Add Slovak language pack

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Synced translations with transifex

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Update changelog

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Update spell checker dictionaries

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Prepare release and bump version numbers to 1.8.9.0

Signed-off-by: Sven Strickroth <email@cs-ware.de>