Valda, don't worry about it - use whatever language people can speak.
If you are nice, people will try to communicate with you. Smile and ask very politely "Do you speak <language>?"

I don't know about Prague precisely because I haven't been there, but in other parts of Eastern Europe it's common that educated people speak Russian, particularly 40+ people. They might on the other hand not speak a word of English - that's quite a common situation.

Prague is a VERY popular tourist destination in the EU, and a rather cosmopolitan city - I think that English is widely spoken there - probably a different situation if you go elsewhere in Czech rep.

I think for younger people they are probably better at English and don't generally speak Russian, with the cut-off somewhere in the mid 30s.

While there are some anti-Russian feelings among some people in Eastern Europe - not all - I think most people take a pragmatic view of language. I think they will be able to tell that you are not a native Russian speaker.

So if you want to practice Russian, approach older people and be super nice, asking very nicely first if they understand Russian. If they say no, try English or German if you know it. It's not unusual that people in parts of Eastern Europe speak German but no English.

Be aware that people might mistake you for a German, particularly if you look like you might be from there. Reason being that Israeli and German accents sound a bit similar in several languages, particularly with the "R".