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.
Always leave %1, %2, %s, etc. in the string, it's a replacement pattern required by the program. For example the string "Constant value of %1" will be expanded to "Constant value of 2000" when setting 2000 RPM value.