If you have all your own plugins then you shouldn't have added any multi-lingual support to them.
What plugin causes the ML_NOT_FOUND? You'll need to remove multilingual support from that plugin.
On a more relevant point, why remove it at all? Just don't let anyone change their language. You can also edit multilingual.amxx and prevent it from showing anything about multilingual support but I'm quite sure that a setting will already do this for you.