Hello @Arianna Mercuriali , Thanks for using Microsoft Q&A Platform.
In the documentation the different source language other than English is also listed for the language pairs please check. Also, German and French as source language is supported; https://video2.skills-academy.com/en-us/azure/ai-services/translator/language-support#custom-translator-language-pairs
I agree with the few custom translator languages supported or not in the language pairs. As per the documentation these are the available language pairs we have at this moment.
I hope this helps.
Regards,
Vasavi
-Please kindly accept the answer and vote 'yes' if you feel helpful to support the community, thanks.