Please add the English language (British English) rather than American only. The correct spelling should be included, such as colour, visualise and cheque.
Thank you for your feedback, we appreciate it and will consider adding a separate British English option in future updates.
That was a fast response!
Actually, in the case of using the web interface, it is the browser dictionary that needs to be changed.
I just changed Firefox to English-Australian and it is doing the correct spell checking now.
The spell checking is indeed not up to us, each browser will use the one it's set to, Todoist itself doesn't have an internal spell checking system, but we'll look into changing the UI accordingly.