Gulden installer language

Gulden installer language is in English on Dutch windows computers.

Make nsis offer a language choice.

Relevant links to implement this for future reference:

http://nsis.sourceforge.net/Creating_language_files_and_integrating_with_MUI https://stackoverflow.com/questions/31992219/how-to-set-a-default-language-on-select-language-dialog

This should be fixed at the same time as #40