Added Slovak language localization.

Based on the en_GB translation. Copied Plural-Forms from pl since it's
similar, not sure if identical, though.

Note that date format at uistr.py:42 seemed to be outdated (didn't
translate correctly), so I used the format from uistr.py source, which
makes the translation work. This may need to be fixed in other translations,
too. Also, some of the line numbers seem to be
outdated.