Dictionary plugin in Sigil? For example Oxford-English Dictionary.
 

Hi. I'm new to Sigil and how plugins work but are trying to learn. :)

Is there a plugin or another way to choose a specific online dictionary in Sigil?

I'm not a native English speaker so when I'm writing in English I sometimes mix US and UK spellings in the same text. I have noticed that when I use "spellcheck" in Sigil set to EN-GB it doesnt highlight all the misspelled words.

Some modern UK spellings are the same as in US but when I'm writing a dialogue set in the Regency period in England for example I would like to use more old-fashioned spelling.

Or if there is another way to do this that I have not thought of I would be very glad for some suggestions. :)

You can add any Hunspell dictionary into the configuration folder:dictionaries

I just dont know where you can get English, middle or English, old dictionaries (ISO 639-2)

Both Sigil and the plugins support hunspell/Myspell spellchecking dictionaries. If you have repeated needs you should create your own wordlists and process them into a hunspell dictionary.