Fix single-quoted strings

In JOSM ' is a special character, so to print an actual ', you have to use ''. In Osmose, the character is not special.

In order to not get strange translations (such as
> unusual value of width: use '' for foot and " for inches, no spaces
instead of
> unusual value of width: use ' for foot and " for inches, no spaces
) we have to convert '' to '. This is necessary as the translated strings obtained from JOSM contain the double '' where ' should be the output.

mapcss/mapcss_lib.py

@@ -445,7 +445,10 @@ def JOSM_search(string):
 #    translate from English to the current language (only for strings in the JOSM user interface) [since 6506]
 def tr(string, *args):
     if string is not None:
-        return T_(string, *args)
+        # Treat '' as ' so JOSM translations work in Osmose too.
+        # A ' is a special character in JOSM, see https://josm.openstreetmap.de/wiki/Translations
+        t = T_(string, *args)
+        return {k: t[k].replace("''", "'") for k in t.keys()}
 
 #regexp_test(regexp, string)
 #    test if string matches pattern regexp [since 5699]