The Constitution doesn't prescribe an official language. I'm not positive but it might have something to do with the fact that the original colonies were multi-lingual. Granted, English evolved into the common language but that was after the founders didn't find it necessary to make it a Constitutional requirement.
They also forgot to establish a national culture, religion or values system within the Constitution or Bill of Rights.
So under what authority do we take down signs not written in English...isn't that an immigrant language also?