Hi there Christian,
Greetings and thanks for your question, sorry about the delay.
Here are some instructions on translating this plugin :)
1. First you'll need to download and install Poedit. You can get it here: http://www.poedit.net/
2. Download MarketPress Lite and unzip the file. Take a look in the languages folder. You'll see a *.po file. This is the language template file provided by the plugin developer to allow it to be translated by WordPress
3. Run Poedit, and open up that .po file. You will see the list of every text that is included with the plugin.
4. Go through the list, and for each time you wish to change something, first click it in the list. You will see the "Source text" box update. Now just enter what you wish to be included instead of that text in the translation box.
It's a long list, so to quickly find instances of "store" be sure to use the "find" function (cmd+F on Mac, and ctrl+F on PC)
5. When you're done translating, click "Validate" to make sure there aren't any errors.
6. Now save the file. This will automatically create a .mo file. This is the compiled version of your translation that WordPress can read faster than the full text version.
7. Rename the .mo file to include the country code. For example, with MarketPress it would be "mp-en_US.mo" Except replace the en_US part with whatever language code you need. You can find references to the codes here: http://codex.wordpress.org/WordPress_in_Your_Language
8. Upload to your site and place in the languages folder of MarketPress (wp-content/plugins/wordpress-ecommerce/marketpress-includes/languages)
9. Tell WordPress to look for translations. This can be done by placing the following line in your wp-config.php file:
As you did before when renaming the file, replace the en_US part with whatever language code you need. You can find references to the codes here: http://codex.wordpress.org/WordPress_in_Your_Language
Hope this helps! :)