Recommended way to translate the app is creating a pull request to its GitHub repository. Otherwise you can use the traditional approach below.
You will need a text editor that supports editing UTF-8 formatted XML files (you can use free Brackets app).
Be careful to escape HTML special characters, some strings use them to display hyper links or bold text, for example:
<b>Open</b> or <b>drag</b> an image to convert or extract
should be displayed in XML as
<b>Open</b> or <b>drag</b>
Always leave %1 (%s) in the string, it's a replacement pattern required by the program. For example the string "Package %1 opened" will be expanded to "Package firefox4pre opened" when opening Firefox browser package.