In Mastodon, you can enable content translation using a DeepL or LibreTranslate backend. This means you need an API key from one of those services.
Once you have an API key, you can email firstname.lastname@example.org to request that your API key be added to your server configuration.
For DeepL, include your API key and if the key is from a free or paid plan.
For LibreTranslate, include your API key and the LibreTranslate endpoint.
You can get a free or paid DeepL API key at https://www.deepl.com/pro-api?cta=menu-pro-api
You can get a paid LibreTranslate API key at https://portal.libretranslate.com/.
You can also run your LibreTranslate server and use it as your translation backend for Mastodon. You can read more about running your own LibreTranslate server at https://github.com/LibreTranslate/LibreTranslate.