Hello,
if you are using the Azure API, the available languages are retrieved directly from the Azure translation service. You cannot add additional ones because it would be impossible for Azure to do the translation.
On the other hand, if you are using the OpenAi API, if you need to use a particular language for your alt text, you can simply edit the prompt asking to create the alt text in the language you want.
I hope I have answered your question. In case I have not, please provide me with more details.
Valerio
