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

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?