Seconding language transfer that someone mentioned, I found it to be hands down the best free resource for Greek.
Assimil courses are also some of the best materials out there, I highly highly recommend it if you can find a course for your target language.
As for apps, I use Clozemaster for drilling words (I use a premium account), but you can make your own anki decks for that purpose.
But nothing beats talking to native speakers, either using something like italki for actual lessons, or just finding random native speakers. I often forget words I repeated 1000 times during my studies, but words I learned by speaking to others somehow just get glued to my mind.
Idk, quite a few (me included) made alt accounts for the time being, and we'll return when it becomes possible, especially if we really do migrate to piefed.