I’ve used Language Transfer with good success. Don’t even need to sign up, you can just go, click on a course and start streaming lessons. You can also download them locally.
https://www.languagetransfer.org/free-courses-1
The idea is to create rules that help you “transfer” words from the source language into the target language, hence, “Language Transfer”.
For example, going from English to Spanish: Words in English that end in ation, will end in acion in Spanish.
Confirmation -> Confirmación Conversation -> Conversación
Further, words following this rule are “ar” verbs. Confirmation -> Confirmación -> confirmar
Another one is words ending in al, which came to Spanish and English via Arabic… are the same. Just said with an accent.
Normal -> Normal Formal -> Formal
A few rules might get you a few hundred words. And while some words might be more formal that how something is typically said, you should still be understood.
They’re completely donation based, ad-free, and no sign-up required.
Looks like you may have made a decision already, but wanted to give Polaris a mention. Been using that on my home server, has a nice Web-UI and a mobile app on F-Droid as well.
https://github.com/agersant/polaris