Add language setting

It's annoying if we must edit ```minetest.conf``` just for change the language, so i add this setting. Although, we still need to restart the game.

Hope this helps!
This is unacceptable.  Not only is it absurdly slow and hacky, but it also misses extended language codes like `pt_BR`.  Add something like this to the main menu API:
```C++
int ScriptApiMainMenu::l_get_available_languages(lua_State *L) {
	std::vector<fs::DirListNode> dir_list = fs::GetDirListing(porting::path_share + DIR_DELIM + "po")
	lua_createtable(L, 0, dir_list.size());
	for (std::vector<fs::DirListNode>::iterator iter = dir_list.begin(), int i = 1;
			iter != dir_list.end();
			++iter, ++i) {
		if (!iter->dir)
			continue;
		lua_pushstring(L, iter->c_str());
		lua_rawseti(L, -2, i);
	}
	return 1;
}
```