If your pull request doesn't get merged in, you still learned just as much as if it did, so the time you spent doing it wasn't wasted. In fact, people may make comments on it explaining why it won't be merged in, so you'll learn more than if it was.

You don't need to be an expert to start contributing—in fact, the best way to improve is to try to contribute, fail, and then learn how to improve next time you tackle and issue :)

Also, your English is very readable. From this comment alone I can assure you it's more than good enough for writing issues and pull requests. You're doing great, just keep at it!

but my English is not fluent

that is one big barrier to entry that we should have at least some focus on trying to solve, depending on the distro you use they might have some communities that speak your language, on fedora we have a bunch of language-specific communities on our Matrix server, where people usually offer to help whenever needed

If you can understand written english well, there's translation/internationalization that isn't too scary. What is your native language?