Good report history, posts and actions that show you understand forum policy, activity (and when you are active could also play a part in it, mods at certain times could be more needed than others), etc. Being a good poster or trustworthy doesn't mean one would make a good moderator.

Finding mods for foreign language boards presents unique challenges, so there's no real set way to go about that. Sometimes it's by polls, sometimes it's just because a certain person has expressed an interest in moderating the board (either by their actions, or by "applying"), etc. It tends to be a more community driven decision, because acting like you know someone because you used google translate on their posts is dumb (though foreign language mods do need to be able to communicate with us in english).

Report a lot of posts and be a very active member of the community and theymos may ask you eventually.