Wow. I've never heard of that language. :) I'm curious: is it like a sub-language or something?
That's because, officially, Serbian and Croatian are different languages, but they're so similar, they're really just dialects of the same language, called Serbo-Croatian.
Is it typical of Polish speakers that the last vowel is so quiet one can hardly hear it? That "on", "ona", and "oni" are all all pronounced nearly identically ("on"), and you can only tell the difference when they are written? Or is it just this speaker?
It is because the accent in Polish the accented syllable is usually the last but one syllable. But I personally can hear the difference clearly, moreover the forms of verb are totally different.
Bear in mind that 'on' and 'oni' differ in more than just the addition of 'i'; the n-consonant in 'on' (and 'one') is a regular (dental) 'n', while that in 'oni' is the palatal sound also represented by 'ń'