War: Feudal Europe 1300-1780

This is very much in progress England and France (100 years)
France and Spain, 1500s
England and France again, 1700s
French & Indian Wars (England and France)

Many, many wars in Feudal Europe occured between countries which spoke different languages, although it can't generally be argued that they were started over linguistic issues, or the peace was resumed for them.

Certainly propaganda played a part in these wars, and stable resolutions of the ones listed above maintained the general linguistic boundaries.