A KOReader user patch to use DeepLs api for translation instead of google translates api. Demonstration: https://imgur.com/a/zTG3EUn

2-use-deepl-for-translation.lua

--SETTINGS--========================================


local api_key = "ENTER API KEY HERE"
local USE_FOR_TRANSLATION = true
local USE_FOR_DETECTION = true
local SHOW_DEEPL_LANGUAGES_AS_OPTIONS = true

-------------=========================




local Translator =  require("ui/translator")
local Device = require("device")
local InfoMessage = require("ui/widget/infomessage")
local TextViewer = require("ui/widget/textviewer")
local UIManager = require("ui/uimanager")
local Screen = require("device").screen
local ffiUtil  = require("ffi/util")
local logger = require("logger")
local T = ffiUtil.template
local _ = require("gettext")



local AUTODETECT_LANGUAGE = "auto"
local DEEPL_SUPPORTED_SOURCE_LANGUAGES  = {
    AR = "Arabic",
    BG = "Bulgarian",
    CS = "Czech",
    DA = "Danish",
    DE = "German",
    EL = "Greek",
    EN = "English (all English variants)",
    ES = "Spanish",
    ET = "Estonian",
    FI = "Finnish",
    FR = "French",
    HU = "Hungarian",
    ID = "Indonesian",
    IT = "Italian",
    JA = "Japanese",
    KO = "Korean",
    LT = "Lithuanian",
    LV = "Latvian",
    NB = "Norwegian Bokmål",
    NL = "Dutch",
    PL = "Polish",
    PT = "Portuguese (all Portuguese variants)",
    RO = "Romanian",
    RU = "Russian",
    SK = "Slovak",
    SL = "Slovenian",
    SV = "Swedish",
    TR = "Turkish",
    UK = "Ukrainian",
    ZH = "Chinese (all Chinese variants)"
}

local DEEPL_SUPPORTED_TARGET_LANGUAGES  = {
    AR = "Arabic",
    BG = "Bulgarian",
    CS = "Czech",
    DA = "Danish",
    DE = "German",
    EL = "Greek",
    EN = "English (unspecified variant for backward compatibility; please select EN-GB or EN-US instead)",
    ["EN-GB"] = "English (British)",
    ["EN-US"] = "English (American)",
    ES = "Spanish",
    ET = "Estonian",
    FI = "Finnish",
    FR = "French",
    HU = "Hungarian",
    ID = "Indonesian",
    IT = "Italian",
    JA = "Japanese",
    KO = "Korean",
    LT = "Lithuanian",
    LV = "Latvian",
    NB = "Norwegian Bokmål",
    NL = "Dutch",
    PL = "Polish",
    PT = "Portuguese (unspecified variant for backward compatibility; please select PT-BR or PT-PT instead)",
    ["PT-BR"] = "Portuguese (Brazilian)",
    ["PT-PT"] = "Portuguese (all Portuguese variants excluding Brazilian Portuguese)",
    RO = "Romanian",
    RU = "Russian",
    SK = "Slovak",
    SL = "Slovenian",
    SV = "Swedish",
    TR = "Turkish",
    UK = "Ukrainian",
    ZH = "Chinese (unspecified variant for backward compatibility; please select ZH-HANS or ZH-HANT instead)",
    ["ZH-HANS"] = "Chinese (Simplified)",
    ["ZH-HANT"] = "Chinese (Traditional)"
}


local function loadDeepLPage(text, target_lang, source_lang)
    local http = require("socket.http")
    local ltn12 = require("ltn12")
    local json = require("json")
    local socketutil = require("socketutil")
    local socket = require("socket")

    local url = "https://api-free.deepl.com/v2/translate"



    -- request_body.target_lang = target_lang
    --request_body.source_lang = source_lang

    local request_body = {
        text = {text},  -- Wrap text in a table (array) for DeepL API
        target_lang = target_lang,
    }

    if source_lang and source_lang ~= "auto" then
        request_body.source_lang = source_lang
    end


    local request_json = json.encode(request_body)
    local response_body = {}

    socketutil:set_timeout()
    local request = {
        url = url,
        method = "POST",
        headers = {
            ["User-Agent"] = "KOReader",
            ["Host"] = "api-free.deepl.com",
            ["Accept"] = "application/json",
            ["Authorization"] = "DeepL-Auth-Key " .. api_key,
            ["Content-Type"] = "application/json",
            ["Content-Length"] = tostring(#request_json)  -- Fix Content-Length
        },
        source = ltn12.source.string(request_json),  -- Send JSON
        sink = ltn12.sink.table(response_body)
    }
    logger.dbg("Calling", request.url)
    -- Skip first argument (body, goes to the sink)
    local res, code, headers, status = http.request(request)

    socketutil:reset_timeout()

    -- raise error message when network is unavailable
    if headers == nil then
        error(status or code or "network unreachable")
    end

    if code ~= 200 then
        logger.warn("translator HTTP status not okay:", status or code or "network unreachable")
        logger.dbg("Response headers:", headers)
        return
    end

    if table.concat(response_body) then
        local response_str = table.concat(response_body)
        local content = json.decode(response_str)
        logger.dbg("translator content:", content)

        if content.translations then
            return content.translations
        else
            logger.warn("translator error")
        end
    else
        logger.warn("not JSON in translator response:", response_body)
    end

end

local detect_original = Translator.detect
Translator.detect = function(self,text)
    if not USE_FOR_DETECTION then
        return detect_original(self,text)
    end

    local result = loadDeepLPage(text, "en", AUTODETECT_LANGUAGE)
    if result and result[1] and result[1].detected_source_language then
        local src_lang = result.detected_source_language
        logger.dbg("detected language:", src_lang)
        return src_lang
    else
        return self.default_lang
    end
end

local translate_original = Translator.translate
Translator.translate = function(self, text, target_lang, source_lang)
    if not USE_FOR_TRANSLATION then
        return translate_original(self, text, target_lang, source_lang)
    end

    if not target_lang then
        target_lang = self:getTargetLanguage()
    end
    if not source_lang then
        source_lang = self:getSourceLanguage()
    end

    if not DEEPL_SUPPORTED_TARGET_LANGUAGES[target_lang] then
        UIManager:show(InfoMessage:new{
            text = _("Language: "..target_lang.." not supported as target language for translation by DeepL")
        })
        return
    end

    if source_lang ~= "auto" and not DEEPL_SUPPORTED_SOURCE_LANGUAGES[source_lang] then
        UIManager:show(InfoMessage:new{
            text = _("Language: "..target_lang.." not supported as source language for translation by DeepL")
        })
        return
    end

    local result = loadDeepLPage(text, target_lang, source_lang)
    if result and result[1] and type(result[1]) == "table" then
        local translated = {}
        for i, r in ipairs(result) do
            table.insert(translated, r.text)
        end
        return table.concat(translated, "")
    end
    return nil
end


local _showTranslation_origianl = Translator._showTranslation
Translator._showTranslation = function(self, text, detailed_view, source_lang, target_lang, from_highlight, index)
    if not USE_FOR_TRANSLATION then
        return _showTranslation_origianl(self, text, detailed_view, source_lang, target_lang, from_highlight, index)
    end
    if not target_lang then
        target_lang = self:getTargetLanguage()
    end
    if not source_lang then
        source_lang = self:getSourceLanguage()
    end

    if not DEEPL_SUPPORTED_TARGET_LANGUAGES[(string.upper(target_lang))] then
        UIManager:show(InfoMessage:new{
            text = _("Language: "..target_lang.." not supported as target language for translation by DeepL")
        })
        return
    end

    if source_lang ~= "auto" and not DEEPL_SUPPORTED_SOURCE_LANGUAGES[(string.upper(source_lang))] then
        UIManager:show(InfoMessage:new{
            text = _("Language: "..target_lang.." not supported as source language for translation by DeepL")
        })
        return
    end


    local Trapper = require("ui/trapper")
    local completed, result = Trapper:dismissableRunInSubprocess(function()
        return loadDeepLPage(text, target_lang, source_lang)
    end, _("Querying translation service…"))
    if not completed then
        UIManager:show(InfoMessage:new{
            text = _("Translation interrupted.")
        })
        return
    end
    if not result or type(result) ~= "table" then
        UIManager:show(InfoMessage:new{
            text = _("Translation failed.")
        })
        return
    end

    if result[1] and result[1].detected_source_language then
        source_lang = result[1].detected_source_language
    end
    local output = {}
    local text_main = ""

    local function is_result_valid(res)
        return res and type(res) == "table" and #res > 0
    end

    -- For both main and alternate translations, we may get multiple slices
    -- of the original text and its translations.
    if is_result_valid(result) then
        -- Main translation: we can make a single string from the multiple parts
        -- for easier quick reading
        local source = {}
        local translated = {}
        local romanized = {}
        for i, r in ipairs(result) do
            if detailed_view then
                local s = type(text) == "string" and text or ""
                table.insert(source, s)
                if type(r.romanized) == "string" then
                    table.insert(romanized, r.romanized)
                end
            end
            local t = type(r.text) == "string" and r.text or ""
            table.insert(translated, t)
        end
        text_main = table.concat(translated, " ")
        if detailed_view then
            text_main = "● " .. text_main
            table.insert(output, "▣ " .. table.concat(source, " "))
            if #romanized > 0 then
                table.insert(output, table.concat(romanized, " "))
            end
        end
        table.insert(output, text_main)
    end

    -- table.insert(output, require("dump")(result)) -- for debugging
    local text_all = table.concat(output, "\n")

    local textviewer, height, buttons_table, close_callback
    if detailed_view then
        height = math.floor(Screen:getHeight() * 0.8)
        buttons_table = {}
        if from_highlight then
            local ui = require("apps/reader/readerui").instance
            table.insert(buttons_table,
                {
                    {
                        text = _("Save main translation to note"),
                        callback = function()
                            UIManager:close(textviewer)
                            UIManager:close(ui.highlight.highlight_dialog)
                            ui.highlight.highlight_dialog = nil
                            if index then
                                ui.highlight:editNote(index, false, text_main)
                            else
                                ui.highlight:addNote(text_main)
                            end
                        end,
                    },
                    {
                        text = _("Save all to note"),
                        callback = function()
                            UIManager:close(textviewer)
                            UIManager:close(ui.highlight.highlight_dialog)
                            ui.highlight.highlight_dialog = nil
                            if index then
                                ui.highlight:editNote(index, false, text_all)
                            else
                                ui.highlight:addNote(text_all)
                            end
                        end,
                    },
                }
            )
            close_callback = function()
                if not ui.highlight.highlight_dialog then
                    ui.highlight:clear()
                end
            end
        end
        if Device:hasClipboard() then
            table.insert(buttons_table,
                {
                    {
                        text = _("Copy main translation"),
                        callback = function()
                            Device.input.setClipboardText(text_main)
                        end,
                    },
                    {
                        text = _("Copy all"),
                        callback = function()
                            Device.input.setClipboardText(text_all)
                        end,
                    },
                }
            )
        end
    end

    textviewer = TextViewer:new{
        title = T(_("Translation from %1"), self:getLanguageName(source_lang, "?")),
        title_multilines = true,
            -- Showing the translation target language in this title may make
            -- it quite long and wrapped, taking valuable vertical spacing
        text = text_all,
        text_type = "lookup",
        height = height,
        add_default_buttons = true,
        buttons_table = buttons_table,
        close_callback = close_callback,
    }
    UIManager:show(textviewer)
end


local genSettingsMenu_original = Translator.genSettingsMenu

Translator.genSettingsMenu = function (self)

    local menu = genSettingsMenu_original(self)

    if not SHOW_DEEPL_LANGUAGES_AS_OPTIONS then
        return menu
    end

    local function genLanguagesItems(setting_name, default_checked_item)
        local items_table = {}
        for lang_key, lang_name in ffiUtil.orderedPairs(DEEPL_SUPPORTED_TARGET_LANGUAGES) do
            table.insert(items_table, {
                text_func = function()
                    return T("%1 (%2)", lang_name, lang_key)
                end,
                checked_func = function()
                    return lang_key == (G_reader_settings:readSetting(setting_name) or default_checked_item)
                end,
                callback = function()
                    G_reader_settings:saveSetting(setting_name, lang_key)
                end,
            })
        end
        return items_table
    end

    menu.sub_item_table[#menu.sub_item_table].sub_item_table = genLanguagesItems("translator_to_language", self:getTargetLanguage())
    table.insert(menu.sub_item_table, {
        text = "Currently using DeepL API for translations"
    })

    return menu
end

I'm having a "translation error" message when I enter my API key. How do I fix it?

I put the API key only on line 5; do I have to write it on another line, too? Should I copy and paste the "-fx" characters of the key? Thanks in advance!

I'm having a "translation error" message when I enter my API key. How do I fix it?

I put the API key only on line 5; do I have to write it on another line, too? Should I copy and paste the "-fx" characters of the key? Thanks in advance!

Yes it should be the full api key, including whatever is at the end, in my api key there' as :fx at the end of my key and it works but your one may differ. Just copy and paste straight from your deepl api dashboard and make sure line 5 is like this:
local api_key = "ABC-DEF-GHI:fx"

Worked it! Thank you! I'm native Portuguese brazilian speaker so to translate from en to PT_BR was giving me the error. I changed to PT and worked. Anyway how I can make PT_BR translation work?

Worked it! Thank you! I'm native Portuguese brazilian speaker so to translate from en to PT_BR was giving me the error. I changed to PT and worked. Anyway how I can make PT_BR translation work?

I've updated the code, I think it was just the wrong language code used (it's actually PT-BR)
Just copy and paste the new version of the user patch, go into the menu and reselect Portuguese Brazilian again to store the correct language code and it should work.

Worked it like a charm, thank you for the update!

Thank you!

On my Android device (Boox Go Color 7), it translates fine, but KOReader instantly freezes as soon as the result comes up. I have to force quit it to proceed reading. Any ideas why?