Time localisation - "@count hour" (ago) instead of "1 hour"?
The change you have made should work until you download and install an update to the drupal core language file.

The Drupal Localize site would allow you to download an check the latest versions of the  core language files to see if the bug also exists latest versions of 6, 7 and 8. 

https://localize.drupal.org/translate/languages/cs

Perhaps you could sign up and posting a message into the Czech language forum on the localize site.  You can also find out more about contributing on the localize website.

https://localize.drupal.org