Cosmos/FTS: remove hard-coded fallback to default language for the TFS property if none was specified explicitly

Cosmos FTS should allow for the property language to not be specified (and that should use default language for container), but current version of SDK requires the language to be set. We are manually falling back to container default language (and "en-US" if that wasn't set) for now, until SDK is updated.

Once SDK allows optional language, we can remove this and rely on Cosmos to determine the language.