Can you understand swiss german, bavarian and swabian if you speak standard german?
The answer is yes and no or somehow a little bit. It depends on your High German and if you practice the language in Germany/Swiss/Austria or another German speaking Country. And it depends on the person who speaks dialect. Let's call him Peter. If he is from a bigger town he probaply will speak High German and not so hard the dialect. If he is from a village it will be more difficult. Dialect is more spoken in the country than in big towns. If Peter is from a village near Hamburg and you learned German in Nürnberg it will be harder than you learned it in Hamburg.
But here are the good news. All Germans learn High German at school and they (almost) all learn English at school. So concentrate your practice on High German and don't think too much about the dialects. First the duty then the freestyle. And in the worst case you have your hands and a dictionary to understand Peter.
Best regards Angel
There are enough differences in pronunciation and some different vocabulary, so that you should learn the one you are interested in.
I am a native German speaker and speak mostly standard german. I grew up around Berlin and if I'm talking to someone with a very strong swiss, bavarian or swabian dialect I'm having trouble to understand them. So, if the dialect is strong, it can be pretty hard. But most people can at least "reduce" their dialect, so that it should be possible to talk to each other ;)