To work with translation files, usually you would take a text editor for translation or a specific editor like PoEdit, which have to be installed on your computer first. Using such an editor you can translate *.po files and generate the resulting *.mo files.
With WordPress you can use a plugin to do that, no need to install any software on your machine, it’s quick and save you time.
Codestyling Localization (old)
That was the plugin, unfortunately this solution is no longer available due to compatibility with the new versions of WordPress.
The solution is now:
It does the same job and can be downloaded at https://wordpress.org/plugins/loco-translate/
Click on your theme to get your piece of text and translate them directly into your WordPress admin interface.