BTW There are so many strings just containing a character:
Are there languages which change these characters?
I don't think so, you can consider it as a low priority, translating it or not won't make a difference.
Or this string which is just a non-breakable space:
Same as above, not ideal, but probably also low priority.
I don't get it. Why it is there? For what?
It was not created to drive you crazy, I promise. :) Probably a side effect of a code not being properly internationalized. There are many strings and many lines of code, so those little things always slip in.
An what about HTML failures in strings?
I found it just today and already reported it, thanks. The code that uses that string probably does not seen to come up often. If you see HTML errors like that one, feel free to report here.