How to query entries written in a specific locale? (Craft 3)
In Craft 3, you can't really have locales. If in Craft 2 you had multiple locales, that should have been automatically migrated to multiple sites (as now the site is always single locale)

Having said that, you now need to use site or siteId parameter https://docs.craftcms.com/v3/dev/element-queries/entry-queries.html#site