How to see all Czech translations that have not been yet translated to Slovak?
Special:SearchTranslations should help, but why not focus on untranslated messages regardless of the status in other languages?
Special:SearchTranslations works only when you know the text of the translation but I need the very same interface as when I translate random messages from English to (language I choose). What would happen if I didn't choose English in the beginning? Then it wouldn't be able to show me English translations as the source. I want to achieve something like that, the source would be Czech. I thought I can change/add languages I know also after the registration but I haven't found anything in preferences...
Why translate Czech translations at first? Because I firstly care about what translations are (mostly) seen, then about the others which might not be seen that often. (As I said, Czech is a fallback language for Slovak and I first want to make sure that Slovak texts (not the Czech ones) are appearing on Slovak projects.)
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).