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 than how something is typically said, you should still be understood.

They're completely donation based, ad-free, and no sign-up required.