Rated 5 out of 5 stars

(edit)

Re-installing the plug-in by downloading it through Firefox and manually adding it to Thunderbird resolved the problem.

Before I removed it, it was actually under 'extensions' and now it's under 'dictionaries'.

I do not remember how I installed it (I don't think I tried to install the language pack as the extension I removed was labeled 'woordenboek Nederlands') before but I've reproduced both ways that I could have (through Thunderbird and manually) and they now both yield the expected result.

(Thanks)

This review is for a previous version of the add-on (3.1.3.1webext). 

Hm, strange. Can't test on Linux right now, but Windows seems to work right.
Are you sure you install the dictionary and not the Language pack? Or are you installing from within Thunderbird or from file?

Thanks for your report and feedback. Glad it's working now.