First note that I don't know much about the spellcheck plugin so this is just a speculation.
When you concatenated the two dictionaries, what encoding did you use for the file that contains both of them? To be sure you use the same encoding as the original Greek dictionary, I'd suggest the following: 1. Make a copy of the original Greek dictionary which you'll use as the combined Greek-English dictionary 2. Open it in Geany. Make sure that Greek words look alright - if not, use "File->Reload as" and select the right encoding. 3. Open the English dictionary, select all lines, copy them, and paste them to the file containing the Greek dictionary 4. Check if such a combined dictionary works for you