Johann Leo Weisgerber was a Lorraine-born German linguist who also specialized in Celtic linguistics. He developed the "organicist" or "relativist" theory that different languages produce different experiences. He was the son of a village teacher who served as a young man in the German army in Flanders, so could not return to his home city. During World War II his pan-Celticist ideology was co-opted to support the German war effort, as did pro-Polish and pro-Czech ideology on the side of the allies.

The idea of linguistic relativity, known also as the Whorf hypothesis, the Sapir–Whorf hypothesis, or Whorfianism, is a principle suggesting that the structure of a language influences its speakers' worldview or cognition, and thus individuals' languages determine or influence their perceptions of the world.