Support country-independent languages

The Dutch language is identical for the following countries or regions:
- nl_AN.lm
- nl_AW.lm
- nl_BE.lm
- nl_NL.lm
- nl_SR.lm

Of course, there are local differences in the usage of the language, but there is only one official language and the list contains words which are also specific in usage for certain countries.

Onboard would be able to work with only one file called nl.lm which automatically should be used for all these countries. This will also be of use for other languages.

Implementing this would make maintaining language support easier, would provide language support for all country codes beginning with 'nl'.