English is the main language in server, we use it because is more popular around the world, and most of players can use it at least at basics of language.
We don't force players to use english or other language, they use that they want, but keeping in mind that using english you can make that a mayority understand you, if this is that you wish ofc.
Normally we can't punish for bad words in a language that we can't understand instantly (while we are online), but if some players complain, we can read logs later and translate that we need to judge and punish if/who deserve.
For english, you will see an admin job working most of times online (example muting him and or kick later, or other higher punish depending of happened), because is basic/minimum language that all admins must know at least to get this rank, and they can judge easily.
Also maybe you can see some other admins doing this job with other languages, but this is only because these admins know that language (is native for them) in addition to english... some examples are russian+Percyvelle admin, or turk+Aratar admin.