DeepL: unsupported languages

**Describe the bug**
A misleading error message (400: Bad Request) is returned when unsupported languages are used with the DeepL API. Since DeepL only supports a rather small subset of languages, it would be nice to be informed when a selected target language was unsupported by the API.

There is an endpoint to fetch supported langs documented here: https://www.deepl.com/docs-api/other-functions/listing-supported-languages/

**To Reproduce**
Attempt to translate text from any language to an unsupported one (e.g. Korean [ko]). The error message displayed is: `DeepL: Response status code does not indicate success: 400 (Bad Request)`

**Expected behavior**
A more helpful message should be shown (e.g. `The target language is not supported by this API`)