Does anything say that you have to write in English there? I can't find anything that says so.
Presumably there are Americans who don't know English, yet have money and write checks.
Could I legally write "treinta y tres dólares y ocho centavos" or "dreiunddreissig Dollars und acht Cents"?
Does my bank set policy?
Does the federal government banks set policy?
Does the bank of the check recipient set policy?