Hello Toby,
Unfortunately, this functionality is not available for JIRA right now.
It was requested before as you can see in this Feature request, however, it was not implemented because there are several Browser plugins that already provide it and due to the several numbers of JIRA translations it would prove very difficult to be implemented.
That being said, we recommend you to use one of those plugins such Grammarly.
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.