In spellchecker.html for 2.0.5 documentation (check the last parenthesis):
"In case you have a large body of text already translated, but then realize the target language code of the project does not match the spell checker's language code (you specified pt_BR as the language, but there's no pt_BR vocabularies, for instance) you can simply copy the two corresponding files and rename them (from pt_PT.arr resp. pt_PT.dic to pt._BR.arr and pt_BR.dic)."
--> AFAIK, dictionary files have extensions .aff and .dic. I think the ".arr" extension mentioned should be changed to ".aff"
Fixed in SVN (2.0.5 and /trunk).
Didier
Closing...
This feature was implemented in the just released version 2.0.5 update 1 of OmegaT.