Polish translated messages

Consistent casing, add contexts where needed.
3 jobs for master in 12 minutes and 37 seconds (queued for 32 minutes and 14 seconds)
Status Name Job ID Coverage
  Build
passed build_clazy_clang_tidy #107576

00:09:26

passed linux kf5-qt5 qt5.15 #107575

00:02:40

73.0%
 
  Test
passed reuse #107577

00:00:30