I'm in Slovakia right now and I noticed that Slovak sounds "wrong" to me, in a way other foreign languages don't, even Czech, which is also similar to Polish.

Now I know why. Slovak sounds like foreigners trying to pronounce Polish. Where we have "ł", which is pronounced like "w", they use "l", which is usually how foreigners try to read that letter. Where we have "ś", they usually have "s", etc. Many of the words that are similar in both languages are basically the same, if you took all the "softened" phonemes and hardened them.