I am using InDesign CC 2015 on my Mac with El Capitan. I installed it via CC Desktop Application. Within the InDesign Preferences, I have "English UK" set as my dictionary. However, InDesign is underlining and picking up on words such as 'colour' and 'aluminium' spelt correctly as per UK English and then suggests they change to US English. Do I need to change the language in a second place? Or is it just not a great UK English dictionary?
In the creative cloud desktop application preferences, the App Language is set on 'North American' but I believe that is to do with the language for menus etc. Besides if I try to change that, the application reckons that I have no apps installed.
What determines what language is spell-checked (also hyphenation) is the language attribute you have set for the text in the Control panel or the Character panel (or in your styles). For example, open a paragraph style in the Paragraph Styles panel, and examine Advanced Character Formats > Language.
Many thanks Steve, that solved it for me.