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There is no one Celtic language. Celtic languages (with an 's') are a group of languages ariginating in Europe. Germanic languages are a group of languages also originating in Europe and include...
The Celtic languages are part of a group called the Italo-Celtic branch of the Indo-European family of languages, which arose somewhere in Eastern Europe.
The modern Celtic languages are: Irish Scottish Gaelic Manx Welsh Breton Cornish and various extinct languages: Gaulish, Celtiberian, Lepontic, Galatian, etc.