"html" tag's xml:lang always "en" for all languages
Changing the user's interface language does not affect the xml:lang attribute value of the "html" tag.
Redmine tells the browser that the content is always English, although it's actually not. This way, services such as spell checking, tts, etc can't function correctly for languages other than English.
Redmine should set the xml:lang attribute according to the current user's language setting.