Fascinating, I did not know about Gacería! For compatibility reasons, OSM’s name:* keys follow the IETF’s BCP 47 standard, which uses a combination of ISO 639 language codes and ISO 3166 country codes to identify dialects. In some cases, well-known language variants have been registered with the IANA as official language subtags, but this doesn’t appear to have been done for Gacería. As a last resort, you can use a private use subtag such as name:es-x-gaceria (assuming it’s more closely related to Spanish than to French or Galician).
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