With the greatest of respect to the respondents here (and I do have oodles of respect for volunteers that are trying to help), the OP is clearly asking for advice about the ability to:
- Eliminate the use of US English in all cases, and in every circumstance, at all levels and AT ALL TIMES.
It is so offensive to those of us that are not American, but live and breath English as our first language, that most if not all of our documents default to US English despite our repeated attempts to avoid that. On average, it's not uncommon to have up to 100 spelling and grammar errors (for our audience) when this occurs. It is unprofessional and ridiculously counter-productive.
I appreciate the highly technical answers such as editing custom dictionaries and use of Macros as suggestions, but for some of us, that just isn't seen as a safe option as we are simply not sufficiently technical to trust ourselves to achieve those.
And I also appreciate the claim that Microsoft isn't listening here. In fact, Microsoft simply aren't ever listening anywhere. No amount of complaints and feedback submissions has caused them to think that this simplest of features is a priority. I know, because I've been trying for more then 16 years on this very point.
Again, thanks to the volunteers who have tried to help. But this won't ever be fixed in a simple way. Microsoft just don't care enough.