The API has worked for me for several years but just stopped a few days ago.
I tried this example at https://languagecloud.sdl.com/lc/t/72925/api-documentation:
curl -X POST -H "Content-type: application/json" -H "Authorization: LC apiKey=<YOUR API KEY>" -d '{"text":"Hello%20Developers", "from":"eng", "to":"fra"}' https://lc-api.sdl.com/translate
Of course I replaced <YOUR API KEY> with my API key.
The result was:
{"message":"LanguagePair with eng->fra not found","errorCode":"VALIDATION_EXCEPTION","details":[{"name":"LanguagePair","code":"not.found","value":"eng->fra"}]}