Why hide boards? Users from the German boards can write in the English boards only in English, so that's not a problem. And vice versa of course

. If you see a post that's not in English in the English boards, warn the user and/or delete the post (a suggestion, other courses of action can be taken). Besides, maybe a German user will get some good info from the English board and vice versa. Even though they may not be particularly that good in writing that board's language, they could use their passive knowledge (reading) or use Google Translate

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The closest thing to what you're asking for is the ignore boards option in user preferences. I don't think they can be preset to ignore certain boards by default (at least not without changing code), but every user can set any board to ignore. That way, when the user hits the "Show unread posts since last visit." button, posts made in the board he has set to ignore will not be shown as unread posts

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Or, you could set them to a predefined user group which shows or hides certain boards. You can do this after registration though, not during registration. I wouldn't do this though because of the reasons posted above. I can only think a few reasons why someone would hide certain boards (forum staff boards being the most used one).