You can translate WP Tabs easily using Poedit App.

Download Poedit

There are several apps available to help you with translating .PO files. PoEdit is a popular one of them, and it’s available on every major platform. You can download it from here.

Start Translating

Open the default wp-expand-tabs-free.pot file from your /wp-content/plugins/wp-expand-tabs-free/languages/ folder. When you save your work PoEdit will generate you a .MO file which will be needed to apply your work.

Name your files properly WordPress recognizes your localization by a special formatting of file names. You will need to include the right language and country codes so it can identify what language is your work made for.

For example, a German localization would have the file names: wp-expand-tabs-free-de_DE.po and wp-expand-tabs-free-de_DE.mo. You must follow this syntax, including the dash, underscore, and lower or uppercase letters.

Apply Your Localization

Copy your newly created .PO and .MO files into the /wp-content/plugins/wp-expand-tabs-free/languages/ folder. WordPress should use them immediately if you named your files properly.

If you want to learn more about how WordPress handles language translation then check out this link: http://codex.wordpress.org/Translating_WordPress