Clean up Cognitive Language - Language pillar readme.md

There are currently 3 top-level blocks trying to declare default tags (there can be only one - and it doesn't make sense with separate SDKs anyway), and we could move the title to a global to avoid repeating it.

https://github.com/Azure/azure-rest-api-specs/blob/main/specification/cognitiveservices/data-plane/Language/readme.md

Currently, though, I don't think we're using this file but want to eventually - not just for autorest, but any other tools that need to parse the different versioned swaggers.

/cc @annatisch @abhahn

Start with specification/cognitiveservices/data-plane/Language/readme.md and inspect its three top-level default-tag blocks and repeated title. Consolidate the default-tag declaration appropriately for the separate SDKs and move the title to a global, leaving one coherent configuration for tools that parse the versioned swaggers.

Written by the indexing model from the issue text.