When I create a question or add a comment on StackExchange sites all text within the textarea gets a red underline because of spellchecking - I use a german brower localization (Firefox). AFAIK if those textareas get a
lang attribute set to 'en' this would be easily fixed. I think this is also an improvement in regards to applying HTML standards. For non-English StackExchange sites
lang could be set to another value.
Another option that I don't prefer would be to use spellcheck="false".
Or the lang attribute could be set to whole page.
I don't know whether this can be seen as a feature-request or as a bug.