You can translate Testimonial Plugin easily using Poedit App.
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.
Open the default testimonial-free.pot file from your /wp-content/plugins/testimonial-free/languages/ folder. When you save your work PoEdit will generate you a.MO file which will be needed to apply for your work.
Name your files properly WordPress recognizes your localization by 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: testimonial-free-de_DE.po and testimonial-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/testimonial-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 this link: http://codex.wordpress.org/Translating_WordPress