Tak (так) is Russian word too. Slavic languages have a lot of same or similar words.
And Pawel is common russian name too.
You know it and I know it, but do you think relic knows it? You can see that they differ, no other squad uses these words. And they also say they are not russian so its out of question. If they were byelorussian they wouldn't say that. And if they were lativian or ukrainian they would fight for the germans... what do you have left?
Also, the word "так" i usually used in different meaning, I never heard or seen it used in russian as confirmation , russians tend to always use "да" for that purpouse (unless it's "так точнo" but that's the idiom, partisans use the word alone like in polish). It's mostly used like english word 'so' ('so cold' <-> "так холодно" <-> "tak zimno").