According to this documentation, Confluence has a specific hierarchy when determining which language should be used for displaying Confluence pages. Part of this hierarchy relies on the language setting as stored by a users browser. Currently, the system may return the wrong language when there are multiple flavours of the same language installed e.g. if English US and English UK are installed and English US is requested, English UK may be returned.
- Set the system language to English UK (default)
- Create a user with no language preference in a Confluence instance
- Log in to your user profile and click settings and note that the language settings display "Automatically Detect Browser Setting"
- Change the language setting in your browser to English (US)
Since you have not specified another language on your User Profile or via cookie, Confluence will detect this language setting via browser and display Confluence pages in English US
Confluence serves the page in English UK
A possible workaround has been notated in this KB Article: Confluence and Team Calendars for Confluence Do Not Detect Browser Language Preference