Setting the policy (as recommended only) moves recommended locales for a managed session to the top of the list, in the order in which they appear in the policy. The first recommended locale is preselected.
If not set, the current UI locale is preselected.
For more than one recommended locale, the assumption is that users want to choose among these locales. Locale and keyboard layout selection is prominent when starting a managed session. Otherwise, the assumption is that most users want the preselected locale. Locale and keyboard layout selection is less prominent when starting a managed session.
If you set the policy and turn automatic sign-in on (see the DeviceLocalAccountAutoLoginId and DeviceLocalAccountAutoLoginDelay policies), the managed session uses the first recommended locale and the most popular matching keyboard layout.
The preselected keyboard layout is always the most popular layout matching the preselected locale. Users can always choose any locale supported by Google Chrome OS for their session.