Blank some translation files

These were mis-merged from other languages by some old pootle that
confused project indices. As there had been no actual changes except
the erroneous updates from template, starting at zero should do.

Note that there are other languages where we have a mix of wrong and
right translations right now. Fixing those will require more effort.
Manually prune da.po
Manually prune ja.po
Manually prune el.po
Manually prune hi.po
Manually prune ca@valencia
Merge pull request #2186 from unknown-horizons/awful-pootle

Fix several wrong pootle merges between different languages