Translation overrides is not working

Using how-to about [translation overrides](http://learn.getgrav.org/content/multi-language#translation-overrides) maybe i found an issue. I created _/user/languages/cs.yaml_ with simple content:

`PLUGIN_ADMIN.TITLE: my translation`

but "my translation" will not replace the original translation even if i delete cache. In settings i have _cs_ used as default language.