Remember that romance languages are the merging of latin with the local celtic lenguage. It has been theorized that germania is the name given to these lands by the romans, and not on what they named themselves. So it is possible that the celtic people knew of these people by the name they called themselves (from all- and mann, all men). I know this is a lot of speculation but it is dificult to ascertain when most written sources come after and from the romans