From the kb article for 'FAQ: Customizing, branding and localizing XUI end user pages in AM/OpenAM' under sub-heading for 'How does AM/OpenAM choose the language shown in user-facing XUI pages?'
It shows how the XUI pages on AM is localized:
1.The locale query string parameter in the URL, for example, &locale=fr
2.The locale set in the user’s browser, which is a 2 or 5 character long language or locale, for example en or en-US.
3.The Default locale, which is set to en by default. This is used when neither a query string parameter or browser language is set.
However, it appears that the old legacy UI was leveraging 'preferredLanguage' of the users profile attribute. For this purpose, LDAP schema has been extended
'iPlanetPreferences' AUXILIARY MAY ( preferredLanguage $ preferredLocale $ preferredTimeZone )
AM system honours the preference set in users profile for 'preferedLangauge' so it loads the 'translation.json' after the profile page is rendered. It should leverage preferredLanguage from the user identity subject
AM XUI does not leverage the 'preferredLanguage' of the users profile attribute to select 'translation.json' as was the case in the legacy UI