After I posted my question, I tried adding new translations for some other plugins. It turned out they DO shows their translations.
So I don’t have problems with all plugins. Only the plugins I mentioned in the question above happen to have problems.
I will try to find out what’s wrong with them specifically.
If you can, please post some resources that I can use. Otherwise, I will try to find them myself in the forum’s stickied posts.
Thank you.