The only way to add spell checking for a language is to be specific. I mean, not only adding the language but adding the language for a specific culture or country. In your case yo have added English (generic), but if you want to enable spell checking in this language you should be specific and add, for example English (United States), such as this:
If I've added only "Spanish" I culdn't have enabled the spell checker for that language. However, as soon as I add "Spanish (Spain)" the corresponding checkbox in the Spell Check section is ready to be enabled.