Would it make any sense to buy an expansion in a language I don't understand?
I'm thinking something like a reference sheet, but I guess it would take so much time to look it up.
Another option since I sleeve everything would be to print the text in English and put it over?
Yes, this sounds an excellent idea! The cards have a lot of text on them. You can always learn this language either way. You can print the rules from FFGs official site.