There are several tools for translating, but I will show you how to do this using Poedit.
First, download and install the software. Open Poedit, go to File -> Open and browse to slider-pro/languages/slider_pro.po. Then go to File -> Save As and save the file using a name that corresponds to the language to which you're translating the plugin. For example, if you're translating the plugin to German, save the file as "slider_pro-de_DE"; if you're translating it to French, save the file as "slider_pro-fr_FR".
After saving the file, you're ready to begin translating the plugin. All you have to do is click on the string that you want to translate and write the translation for it in the second box, below the English version.