Thanks ender, Lucien_Lachance, viyoriya and aleksvor,
I think I prefer the solution of aleksvor..., it seems the most save option. But I need a little bit more help.
I installed menulibre and typed "env LANG=nl_NL.UTF-8 grams F%" as command and "/usr/bin/grmaps" for the working Directory. When I hit the test button I get this:
Name=Gramps Genealogy System
Comment=Manage genealogical information, perform genealogical research and analysis
Exec=env LANG=nl_NL.UTF-8 gramps F%
Keywords=Genealogy;Family History;Research;Family Tree;GEDCOM;
But it's not working jet...