Windows – Office 365 in English but changing default editing language

OK finally I figured it out. I'm answering my own question because it can be useful to others. Basically I have two different input methods:

• English (Australia) / US keyboard
• English (Australia) / United States - International keyboard

However, if I am browsing in a website, or open e.g. a Word document, which is in English US, Windows will add a third input method to the Win + Spacebar menu:

• English (United States) / US keyboard

I suppose the reason is that while editing an English-US document, or commenting e.g. in an English-US web forum, you might want to be able to use the spell checker in US English as well, hence that third entry in your menu.

As soon as I close said document, or I leave said website (for example I go back to Google Australia) then the third option disappears automatically. It is interesting that the same thing is not done when I visit an English-UK resource, probably because Australian English and UK English do not feature significant spelling differences between them.

PS as you can see in the comments to my question there was some degree of misunderstanding about languages vs keyboards: here we are talking about input methods where any input method is a combination of a language (which is relevant e.g. for spell checking) and keyboard (e.g. English-US, English International or Dvorak). Above I have used a slash to separate the name of the language from the name of the keyboard.

From Change your display language and time zone in Office 365 for Business (emphasis mine):

'Important: Some Office 365 settings, including display language, can't be changed by users in certain Office 365 configurations. If none of the following procedures work for you, contact your organization's Office 365 administrator for help.'