Hi there! A non-english user here (I'm spanish and my mother tongue are both spanish and catalan).
I use a mix of my own english knowledge (learned in school and practised every time I could) with some Google Translator / Wordreference / similar systems just to check if what I'm trying to say is correct or not.
Sometimes I made mistakes, but I think it's all part of a learning curve and it's a matter of time to improve it: The more you translate, the more you read, the more you know about the language... everything counts and it's a very interesting "incremental" process in wich every pice of knowledge you get helps you in the future.
Anyway, and like some other users suggested previously, one thing is being able to express yourself in a correct way (I mean, in an academic way following all the grammar rules and with all the valid syntax that a language has) and another (and maybe that's a totally different thing) is being able to express like a """native""". There're lots of expressions, constructions and even vocabulary that are right but seems weird when you heard it. It's because the person is speaking in an incorrect way? Nope, he's just saying things you understand but in a way that you don't expect.
At this point, I think a non-english author can follow two different paths:
The first one involves using an external translator: Basically you pay someone else (or talk to a friend or whatever) for translate your comic. If you don't mind about improving your english or you just want the world to enjoy your work without worrying about anything else, maybe that's the right way for you.
The other path, and the one that I'm trying to follow, involves a mixture of improving your english for both your use in a comic and in a "personal" context. It's also a funny and challenging way to learn more about a foreign language and become fluent in it.
And, since english it's not the "language for my comic" but probably the "language of the Internet" (not exactly that, but I think all of you get the point) I really encourage you to choose this second way. The possibilites are endless, don't think only in your comic!