The HTML tag always has lang="en" even if the language in settings is set to a different language
Original ticket: https://github.com/mattermost/mattermost-server/issues/21552
Summary
The html tag always defines the language of the page to <html lang="en">. We want this to be set to the “DefaultClientLocale”, that can be found in the config. If there is no value in the config, we can set it to unknown.
The objective of this is so browsers translate the content of the site (the messages in the posts) from the language the server is in theory using.
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Source: mattermost/mattermost