Thanks for your help.
As you suggested I put The Joomla Languagefilter Plugin before the jDiiction Plugin, but still get the same error. Also the Check now reads
Version Library Version: 1.4.1
Joomla Version 3.4.1 is untested.
Plugin JDiction Plugin is enabled
JDiction Plugin is not the first loaded Plugin
Languagefilter is enabled
Database jDiction Database Driver is selected
Language Seam to be good
Module Module is enabled
Common mistakes It seems that there are no common mistakes.
The problem seems to happen when going from Welsh (Default) to English.