Hannah Yeager
As for English not being your first language, what's wrong with writing a fan fiction in your native tounge? Not to be mean, but I know in my fandom, there is at least one - if not more, who write stories in French and Spanish. Last time I checked, ff.net did have the option for posting stories in languages other than English.
I know this because some of my earlier works were translated into French and got reviews from French writers on the site. There is no shame in writing a fan fiction in your native tounge - even if that native tounge isn't English. No one should be ashamed of writing a story in their native tounge - no one.
I beta for a couple of people whose first language isn't English (I think, in both cases, Spanish?). I think there are a few reasons why they write fanfiction in English apart from shame of their native language. First, the primary language used on FFnet, and indeed in many parts of the internet, is English. By writing something in English, you get more exposure, more pageviews, more popularity. Another reason is to practice English. If they don't live in an area where they can practice English but still want to improve, then writing in that language will help them. Also, one (as far as I know) lives in the States. The main language spoken in the US is English, and a good portion of their peers may speak English and not Spanish, leading them to write in English and not Spanish.
TL;DR I do think that writing in your native tongue is good, especially if you don't have a great hold on English and are just starting out, but there are a few reasons that people would write in a language not their own. (Also, I'm not sure if I've ever seen a fic originally written in French or Spanish or something and then translated to English, rather than vice versa).