Translating plural messages

[flatcap]

@reisradomil has created a Pull-Request to split up a number of translation strings.

This replaces ugly messages like:

• Delete message(s)
  to a pair (or more):
• Delete message
• Delete messages

This is a little more work for the translators, but it results in more natural sounding messages.

To read more about the changes see PR#1156