The Math-o-mir is a non-Unicode software and it relies on Windows to translate codepages. Therefore, to correctly write text in your language, you must correctly set Windows codepage translation settings. This is the problem Ieva Birzleja had, and I am thinking others might have it also.
In Windows XP check: Control Panel -> Regional and Language Options -> Advanced ->Language for non-unicode programs
In Windows Vista: Control Panel -> Reginal and Language Options -> Administrative -> Change System Locale
This will of course affect all your non-unicode programs. (On my opinion Windows makes really good job with this codepage translation… as good, in fact, that I have no much desire to make MoM unicode.)