Change Spelling Language on Google Chrome

I use Google Chrome to write in UK English, most of the time. However, occasionally I need to write in US English. How do I change spelling language in Google Chrome?

Most advice on the Inteet says that I should click 'Use this language for spell checking' in Language settings, but this option doesn't exist in my Chrome. See the below image, the top screenshot is from an article telling me to click 'Use this language for spell checking'. But this option doesn't exist in the lower screenshot, which is my install of Chrome:

I am using Chrome 50.0.2661.86 on OS X 10.11.4.