Essentially, the selected locale information is saved in a cookie in a user's browser - with the value being the 2-character locale representation, for example:
Cookie Name: com.idmlogic.sigma.web.util.CompoundLocaleResolver.LOCAL
In case the cookie is not sent by the browser when visiting Identity Portal, then the server falls back to using the following heuristics to determine the display language:
Priority #1: use the browser language provided that this language is enabled in Identity Portal
Priority #2: use the language defined as "default" in Identity Portal Admin UI configurations.
In some cases, due to specific security configuration on a specific site, we could see that the browser does not store, or perhaps does not sent the relevant cookie, in which case the other 2 priorities will be used to determine the language used.