But who will moderate the content? Who is to say to what is legal and where? In USA and in EU different pieces of information can be shown. CSAM needs to be removed. Main lemmy devs removed only active mod on !anime!anime@lemmy.ml due differences in censorship.
I think short event or campaign with push for donations with a pop up that you actually can dismiss. An ad like banner. The biggest problem would be community organization as Lemmy isn't only decentralized horizontally but also vertically. Different front ends, different apps different instances. Most of them wouldn't want to implement an ad that wouldn't benefit them directly. They also have costs with running their piece of lemmy. So some cut for them should be included.
I think a dedicated trustworthy person should be responsible for organizing this campaign as developer time is best spent elsewhere.
The most important factor in language learning is consistency. So try to find something that is fun and engaging for you. Anki for vocab + textbooks or youtube lectures for grammar is one of the cheapest options.
tomato soup made from yesterdays chicken broth (epilepsy warning)
edit:
or soup from snow