I guess I'll say mine now. Japanese is my second language. I started off many years ago, and like most people, my main interests were in understanding anime, and while I've still got a huge interest in the overseas media, I've sorta developed more of a genuine interest in the language and culture. I prefer watching anime in their original language not because English dubs are bad (they've actually gotten very good in recent years), but because I just like hearing the characters speak in Japanese. I'll even watch the Simpsons or a Disney song in Japanese if I get the opportunity (Youtube's ripe with foreign Disney ^^). That said, it's really no surprise that I'm trying to make my doujinshi in Japanese.
Though, as an added bonus, I can make manga seem more "genuine" just by making it in Japanese first (I, of course, intend to make an English version alongside it). Aside from the fact that several people will shun pseudo anime/manga just because of some inherent bias of non-Japanese media, some things can work exclusively in Japanese, like hierarchy of respect, or the fact that a number of series will abuse English in a rather amusing way (I intend to feature a foreign character myself at some point ^^). Above all, I can probably get the original audience of the franchise (nay, anime and manga in general) to enjoy my work.
On that note, I've been a pretty strict follower of Japanese Pokemon canon for a while now. If one were to look at my since-abandoned script, they'd see that it's nothing like the heavily Americanized franchise. Names, background writing, culture, they're all quite prominently Japanese (though, I also take care to do research and not stereotype things... I'm sure we all can agree not every Japanese guy walks around in a kimono, business suit, or school uniform all day and swings around a katana). No offense to some fic-writers, but it does kinda irk me a bit to see people take a Japanese franchise like this and heavily Americanize it (moreso than the translated official media, as at least that had a Japanese basis... this can be taken to a point where the setting could actually be considered America, even though the game regions in question are based off of Japan). It especially irks me when they "try" to make it look Japanese but utterly fail. They'll make the character read manga and eat ramen one minute, then make some sort of pun that could only work in English or be unfamiliar with a manjuu (or some other common Asian food) the next (thus, they just make their character look like a common western otaku in the end). But I guess it takes a lot of work to write something in one language while trying to portray the culture within the fiction as speaking another language, and some people just aren't willing to relate their stories to the franchise's country of origin. I actually tried doing that in my defunct script (which was in English), but now, since I'm not extremely far off from writing Japanese on an average, comprehensible level (and it's taking a while to get started on my doujinshi anyway), I figure I'm just gonna go all the way with that. Also, if I work on such a project with a Japanese proofreader helping me out, there's potential to increase my proficiency in the language even further, thus more reasons I'm doing this.