Thanks to Rubão for giving me information to solve the problem!
If you have problems with a non working delete key using the webclient and Swedish language this was how I solved it:
Open /opt/zimbra/jetty-6.1.5/webapps/zimbraAdmin/WEB-INF/classes/keys/ZmKeys_sv.properties (or the place your file is located)
Replace every INHERIT = e-post with INHERIT = mail
Replace INHERIT = klickvy with INHERIT = tabView
Replace INHERIT = Global klickvy with INHERIT = global, tabView
Open /opt/zimbra/jetty-6.1.5/webapps/zimbraAdmin/WEB-INF/classes/keys/AjxKeys_sv.properties (or the place your file is located)
Replace every INHERIT = knapp with INHERIT = button
After a zmmailboxdctl restart my delete key was working as intended again.