Bash script using Google Translate API to translate English to Spanish using curl and sed only. You can change the 'sl' and 'tl' (source/target language) query parameter to whatever you want. Optionally if you create a symlink and call it 'en' it will translate back to spanish

es

#!/bin/bash
# uncomment and fix with appropriate values if you are behind a proxy
#export https_proxy='http://localhost:3128'
sl=en
tl=$(basename $0)
if [[ "${tl}" != "es" ]]; then
  sl=es
fi
base_url="https://translate.googleapis.com/translate_a/single?client=gtx&sl=${sl}&tl=${tl}&dt=t&q="
ua='Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.0.2987.133 Safari/537.36'
qry=$( echo $@ | sed -E 's/\s{1,}/\+/g' )
full_url=${base_url}${qry}
response=$(curl -sA "${ua}" "${full_url}")
echo ""
#print only first translation from JSON
echo ${response} | sed 's/","/\n/g' | sed -E 's/\[|\]|"//g' | head -1

Hi. Thanks guys - nice script.
I had a problem with encoding special characters when using source language like slovak, czech, polish... Only change i did was in query string. Instead of using sed (replace space with +) I used jq for complete url encoding. This is my query that works correctly for all special chars:
qry=$( echo $@ | jq -sRr @uri )

Nice will update the gist in a bit. Thanks.

Less dependencies

function gtr {
  sl=en
  tl=$1
  shift
  base_url="https://translate.googleapis.com/translate_a/single?client=gtx&sl=${sl}&tl=${tl}&dt=t&q="
  ua='Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:109.0) Gecko/20100101 Firefox/109.0'
  qry=$( echo $@ | sed -e "s/\ /+/g" )
  full_url=${base_url}${qry}
  response=$(curl -sA "${ua}" "${full_url}")
  echo ${response}  | sed 's/","/\n/g' | sed -E 's/\[|\]|"//g' | head -1
}

Exploit curl more:

gtr() {
  sl=en
  tl=$1
  shift
  qry="$@"
  ua='Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:109.0) Gecko/20100101 Firefox/109.0'
  base_url='https://translate.googleapis.com/translate_a/single'

  resp="$(curl \
    --silent \
    --get \
    --user-agent "$ua" \
    --data client=gtx \
    --data sl="$sl" \
    --data tl="$tl" \
    --data dt=t \
    --data-urlencode q="$qry" \
    "$base_url")"
  echo "$resp" |
      sed 's/","/\n/g' |
      sed -E 's/\[|\]|"//g' |
      head -1
}

I didn't try out this verbose version of command, but did try on curl -sGA "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:109.0) Gecko/20100101 Firefox/109.0" -d client=gtx -d sl=auto -d tl=en -d dt=t --data-urlencode q="hello world" "https://translate.googleapis.com/translate_a/single" and it worked.

function gtr {
  sl='auto'
  tl=$1
  shift
  base_url="https://translate.googleapis.com/translate_a/single?client=gtx&sl=$>
  ua='Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:109.0) Gecko/20100101 Fi>
  qry=$(printf "%s" "$@" | xxd -plain | tr -d '\n' | sed 's/\(..\)/%\1/g')
  full_url=${base_url}${qry}
  response=$(curl -sA "${ua}" "${full_url}")
  echo "${response}" | sed 's/","/\n/g' | sed -E 's/\[|\]|"//g' | head -1
}

for non-english locales