since the old advice did not work for me and I finally found out what to do there's a patch:
Index: MESSAGE =================================================================== RCS file: /cvs/ports/mail/thunderbird-i18n/pkg/MESSAGE,v retrieving revision 1.2 diff -u -r1.2 MESSAGE --- MESSAGE 24 Jul 2011 07:13:01 -0000 1.2 +++ MESSAGE 3 May 2013 08:55:49 -0000 @@ -1,3 +1,3 @@ -To configure Thunderbird for your language, open the configuration -editor (Preferences->Advanced->Config Editor) and set the -"general.useragent.locale" key to the language you want to use. +To configure Thunderbird for your language, go to add-ons language +management (Tools->Add-ons->Languages) and click 'activate' on your +installed language, then restart Thunderbird. Index: MESSAGE =================================================================== RCS file: /cvs/ports/www/firefox-i18n/pkg/MESSAGE,v retrieving revision 1.1.1.1 diff -u -r1.1.1.1 MESSAGE --- MESSAGE 25 Jun 2006 21:25:44 -0000 1.1.1.1 +++ MESSAGE 3 May 2013 08:55:07 -0000 @@ -1,3 +1,3 @@ -To configure Firefox for your language, type "about:config" in the URL -bar and edit "general.useragent.locale" (e.g. fr-FR ; default is en-US), -then restart Firefox. +To configure Firefox for your language, go to add-ons language +management (Tools->Add-ons->Languages) and click 'activate' on your +installed language, then restart Firefox.
Index: MESSAGE =================================================================== RCS file: /cvs/ports/mail/thunderbird-i18n/pkg/MESSAGE,v retrieving revision 1.2 diff -u -r1.2 MESSAGE --- MESSAGE 24 Jul 2011 07:13:01 -0000 1.2 +++ MESSAGE 3 May 2013 08:55:49 -0000 @@ -1,3 +1,3 @@ -To configure Thunderbird for your language, open the configuration -editor (Preferences->Advanced->Config Editor) and set the -"general.useragent.locale" key to the language you want to use. +To configure Thunderbird for your language, go to add-ons language +management (Tools->Add-ons->Languages) and click 'activate' on your +installed language, then restart Thunderbird.
Index: MESSAGE =================================================================== RCS file: /cvs/ports/www/firefox-i18n/pkg/MESSAGE,v retrieving revision 1.1.1.1 diff -u -r1.1.1.1 MESSAGE --- MESSAGE 25 Jun 2006 21:25:44 -0000 1.1.1.1 +++ MESSAGE 3 May 2013 08:55:07 -0000 @@ -1,3 +1,3 @@ -To configure Firefox for your language, type "about:config" in the URL -bar and edit "general.useragent.locale" (e.g. fr-FR ; default is en-US), -then restart Firefox. +To configure Firefox for your language, go to add-ons language +management (Tools->Add-ons->Languages) and click 'activate' on your +installed language, then restart Firefox.
