I think the easiest way to do this is, is by going to language -> language settings and adding a new language here.
Then you can go to language → Batch translate.
This will open a menu where you can add 1 on 1 translations for each label in the app, like in an Excel.
Is this what you are looking for?
You can also export the language. This way you could hand over the Excel to a person that understands both languages (the current language and the language you need to add). We use a translation bureau for this. You can then import each language again after that person give you back the Excel with all the right translations.
See also point 5 in the documentation here: https://docs.mendix.com/howto7/collaboration-requirements-management/translate-your-app-content/
[EDIT] For Mx10 you got even more options: https://docs.mendix.com/appstore/connector/aws/amazon-translate/