Encoding language diversity in TEI: a description of regional and non-standard languages in multilingual diachronic corpora

French minority and regional languages remain under-resourced in digital formats, limiting cultural preservation and computational studies. The COLaF project develops corpora and NLP tools for French and languages spoken in France in synchrony and diachrony. This paper presents its TEI schema, which allows for the representation of linguistic variation across temporal, geographic and sociolinguistic dimensions. Using a combination of <language>, <person> and <place> elements, the schema links text segments to metadata describing speakers, locations and varieties. Glottolog identifiers are used for non-standard languages. This paper advocates for the creation of a non-BCP 47-language attribute in TEI to enable the interoperability and TEI-compliance of corpora working on non-standard languages. The schema supports diverse document types from 17th-century monographs to web forums and has been applied to several corpora.