Hi Fredrik Kvaal,
Currently only .TDF files are supported in RadDictionary. More information on this, including how such files can be created from plain text is available here.
When it comes to OpenOffice dictionaries, we have considered extending the language support in that direction, but we haven't scheduled a date that the feature will be implemented.
There is a PITS issues which you can use to track our development plans on this matter.
PITS Issue ID: 4162
Public Url: http://www.telerik.com/support/pits.aspx#/public/silverlight/4162
All the best,
the Telerik team
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