Hi Florin,
If you want to provide overrides to the core Umbraco language files, you need to add elements to the already existing file for the relevant language located at ~/config/lang/{language}.user.xml.

Create an area with the alias "buttons" then add a key with the alias "showPage" like so:

  <area alias="buttons">
        <key alias="showPage">Save and Preview</key>
  </area>

This would go in the empty languages element.

As these values are not the core CMS files they won't be modified when you upgrade Umbraco but if the Umbraco project does decide to change the key which it associates with this button your translation may not take affect. This is unlikely but please bear this in mind! :)

You can find out more about language files on the Our Umbraco doc page.

I hope this helped. :)

Thanks,
Jamie