I see some strange behaviour when using shopify.i18n.translate().
I’m using this i18n function as it’s also used in a newly created POS grid extension via Shopify CLI.
When used in Dev Preview the correct device language will be used (in my case de).
But the app extension used regulary/app persistance is using the default language (en).
In both cases shopify.locale shows the correct device locale de-DE.