How to customize the list of spell checkers in Opera 12.x without having to set these languages as preferred languages sent to the servers in the HTTP request header(s)?
Right‐click in any form text field or text area :
contextual menu > Dictionaries > Add/Remove Dictionaries…
Then use the same menu when you need to change dictionary (you should also select your most often used as default when it asks).
This will not add the new dictionary language to your preferred languages.