I don't understand the problem. Is there anything uncommon in wearing casually athletic clothes?
How true is the stereotype of Slavs wearing addidas tracksuits everywhere anyway?
I can say how it looks in Poland.
Partially true. Partially, because it is true only for one particular social group and more or less was a little bit thing from the past, rather than from present times. Wearing tracksuits everywhere was mainly occurring phenomenon in the late 90's/00's and mainly in rather lower class/lower-middle from big cities and later on rural areas, and rather people being young.
Usually it was connected (by media) with some pathological groups, a little bit Gopnik-ish but as far as I remember a lot of people were wearing tracksuits while being rather from lower-middle (so basically like most of the Polish middle class back then) than lower class. As for now people also wear tracksuits as everyday cloth but I would say that males now are wearing more diverse clothes, especially in big cities and richer parts of the country. As for the poorer neighborhoods or smaller towns (or bigger but like Łódź) this is definitely still common phenomenon that during totally normal day you can meet a lot of guys in tracksuits, or at least in some sort of sportswear (or sh**t like this
http://blancz.pl/allegro/D4310912-900B-4ECD-BB35-123C7C509DBC.JPEG). But still they wouldn't probably wear tracksuit for the wedding or funeral. So I wouldn't say that tracksuit is thing Slavs wear since you were born til the end of your life.
On the other hand, I casually wear Adidas clothes. Like, 1/3 of all my T-shirts are Adidas. But as far as I noticed it never was dominant brand, contrary to the meme.