I realized that I provided invalid language tags (with underscores), and that caused errors in clients during discovery (that was matrix-authentication-service).

OpenID Connect Core section 5.1

locale string End-User's locale, represented as a BCP47 [RFC5646] language tag. This is typically an ISO 639 Alpha-2 [ISO639] language code in lowercase and an ISO 3166-1 Alpha-2 [ISO3166‑1] country code in uppercase, separated by a dash. For example, en-US or fr-CA. As a compatibility note, some implementations have used an underscore as the separator rather than a dash, for example, en_US; Relying Parties MAY choose to accept this locale syntax as well.

Note that with this PR underscore are rejected by Authlib.
Checklist

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