Define a paragraph style for the footnotes, in the Paragraph Style Options dialog go to the Advanced Character Formats pane, and from the Language drop-down menu select English: USA (or any language that you prefer). Save the style, and name it something like 'footnotes', or whatever you like.
Then go to Type > Document Footnote Options. Under Footnote Formatting select the paragraph style that you just created from the Paragraph Style drop-down menu.
This should work.
But to save this automatically for any future documents that you create, try this:
Close all open documents (make sure all of them are closed, if not it won't work). Then go into Window > Type & Tables > Character, From the Language drop-down menu, select English: USA (or whatever language you prefer). From now on, I believe, it should always be English bu default.