Find the official instructions for making a new language pack here: http://www.getpaint.net/doc/latest/Translations.html
You don't have to go looking for the resx file any more either. With the release of paint.net 4.0.12 the resx file is installed along with the rest of paint.net. Find the resx file in the \resx\ sub folder in the paint.net installation folder (default: C:\Program Files\paint.net\resx\). The file is PaintDotNet.Strings.3.resx.