if you want to localize Mail Viewer to your native language use file MailViewer.en.lng as a source.
But do not overwrite it! Make your own copy with new filename for translation.

After translation finishing change following values in file according to your settings

[precision language file]
languagename=
author=

and post new translated language file (*.lng) to my email mitec@atlas.cz. 

I will check and include it into distribution package.

Thanx.