Since some hours ago, the default language on many (maybe all) Wikimedia wikis has changed to English. I checked Persian and Arabic Wikipedia, they both are in English. Persian Wikipedia is running 1.35.0-wmf.20, which has apparently been deployed on 20 February 2020, when there was no such issue observed.
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Restricted Application added a subscriber: Aklapper. · View Herald TranscriptFeb 25 2020, 9:59 AM2020-02-25 09:59:53 (UTC+0)
Ahmad252 triaged this task as Unbreak Now! priority.Feb 25 2020, 10:00 AM2020-02-25 10:00:07 (UTC+0)
Restricted Application added a subscriber: Liuxinyu970226. · View Herald TranscriptFeb 25 2020, 10:00 AM2020-02-25 10:00:07 (UTC+0)
Pcoombe closed this task as a duplicate of T246071: Interface language using Accept-Language header value instead of $wgLanguageCode.Feb 25 2020, 10:04 AM2020-02-25 10:04:20 (UTC+0)
Restricted Application removed a subscriber: Liuxinyu970226. · View Herald TranscriptFeb 25 2020, 10:04 AM2020-02-25 10:04:21 (UTC+0)