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:
[Desktop Entry]
Version=1.1
Type=Application
Name=Gramps Genealogy System
GenericName=Genealogy System
Comment=Manage genealogical information, perform genealogical research and analysis
Icon=gramps
Exec=env LANG=nl_NL.UTF-8 gramps F%
Path=/usr/bin/gramps
Actions=
MimeType=application/x-gramps;application/x-gedcom;application/x-gramps-package;application/x-gramps-xml;
Categories=GTK;Office;
Keywords=Genealogy;Family History;Research;Family Tree;GEDCOM;
StartupNotify=true
But it's not working jet...