I think it has to do with the paragraph style that's being used. I would check that first.
Your centering is definitely either the paragraph style or your default paragraph settings (you override any style chosen as the current default by making a change to text specifications with no text selected...).
Language is a different kettle of fish. New users commonly think that choosing a dictionary inthe prefs somehow sets the language, but in reality all it is doing is allowing you to choose which dictionary ID will use for each language when there is more than one installed. Language itself is a character-level attribute so that you can assign different languages to different parts of the text inthe same document, allowing you to use the spelling and hyphenation rules appropriate for foreign words.
Check your paragraph styles for the Language specified (it's in the Advanced Character Formats section) and with no text selected also check, and change if necessary, the laguage specified in the dropdown in either the Character panel or Control panelt to set the new default. If you want to change ALL of your UK English to US English (or any other language) you can use Find/Change. Leave the find and change fields blank, but set the Find Format (panel below the change field -- clcik the magnifier icon) to English: UK and set the Change Format (next panel down) to English: USA, then Change All. This will NOT redefine styles or chagne a default, though, only fix text already in the file.