[AndriyKunitsyn]

My mother tongue was a Russian-Ukrainian mix :)

In Russian, some loanwords are not pluralized, because in order to make a plural, we need to know the word’s declension first, judging from words’ ending, gender and sometimes stress. So for loanwords, it can be hard for the “language feeling” to choose a correct declension form, because no nouns with similar ending and gender can be found. And if no form is found, the word is not declined at all. I think that’s the reason some loanwords in Russian have no declension.

This means that no nouns, except for a limited amount of old native irregular nouns, can “bypass” Russian declension tables - loanwords are never loaned together with their plural forms as it happens in English, they are either pluralized as native words or not pluralized at all. So it’s probably a little bit more consistent. (Ukrainian also works this way, declension tables are just different.)

[Youden]

Do you happen to know why there's no plural form for the borrowed nouns ending in "o", like "radio"? Russian treats these words as neuter, which usually replaces the "o" with an "a" to form the plural (e.g. window), why doesn't it do it here?

[AndriyKunitsyn]

I can't say for sure, I'm not a linguist.

But it's probably because of a declension choosing process which I described. Take "ра́дио", for example - it has this "ио" thing in the ending that is really not typical to Russian, so it made people think "hey, this word is weird, we probably shouldn't touch it when speaking", and since a lot of people thought this way, this became codified.

Now, some other loanwords are not that weird, for example "пальто́" - it's not that different from "окно́", or "весло́", or "ремесло́", and in fact, rarely people do decline it, like "пальта́", "у меня есть два пальта́", "у меня есть пятеро па́льт" - but it's rare, and "officially", it also has no declensions, probably to make things simpler.