I don’t believe the show ever explained it. However, I always put it down to intent. If I remember correctly one of the stated reasons the Ancients created the stargate network, was to allow the less advanced races to travel among the stars. They didn’t have their own technology e.g. ships to get there, so the stargate facilitated trade, communication and hopefully understanding.
So if it’s key goal was to allow people to talk with one another, that would be why it had some kind of translation component within it. However, if the intent isn’t to communicate with everyone, then the Ancient technology would be smart enough to determine that and the translator wouldn’t kick in.
When Zelenka is swearing in czech, he isn’t wanting to communicate with anyone, he’s just expressing his frustration. Hence why it’s not translated. When the goa’uld/jaffa yell commands at one another, their intent is only to communicate among themselves, and not allow their ‘prisoners’ to understand their words.
After all there would be times during diplomacy, when the races would want to talk privately among themselves. The Ancient translator would presumably be smart enough to allow that.
