I'm searching for a documentation of how SharePoint processes the display languages that users can configure in their My Site. Which components (services, timer jobs, caches...) of SharePoint are involved and how do they interact when changing this setting? How does a site "know" about the user's preferred language?
Here is a screenshot of the setting (sorry in German, because this darn feature does not work):
Currently I'm debugging a case where the language setting simply is not applied regardless of what the user configures. Language packs are installed, alternative site languages are enabled, we waited hours after changing the setting. No UI language change. So I want to understand the underlying mechanisms to know where to start looking.
My specific case is a SP 2013 on-prem environment but I assume the underlying architecture did not change much in SPO (at least for the classic experience) given that the classic My Site UI is still there. (It just got more complicated.)
Can anybody provide information or pointers to documentation/books where the architecture of this specific "display language" functionality is described? Or at least provide some insights as to why it might take an hour to apply this setting after making changes?