How to see all Czech translations that have not been yet translated to Slovak?
Right, I had forgotten that we didn't finish the part of the work which involved adding a different source language. The interface is designed to allow it, though, so if you have some programming skills it might be easier to send a patch for that than to use manual methods.
What makes you think that the strings translated into Czech are the most seen? After the group of "top"/most used messages is translated, we have the main Wikimedia messages group which offers plenty of suggestions. Some of those messages are very important, for instance the new search features and notifications which are very visible.
I don't have programming skills to do that so could you please recommend me somebody who would be able to help me?
Seeing those Czech messages on Slovak projects makes me think that they are most seen.
Maybe, if I can suggest something, if it is not too hard and time-consuming, adding an option to switch the source language would be a very useful thing (at least for me).