Primary Localization is the default language of a storefront — the metadata most users in that country see, and the first localization the App Store indexes for keyword search there.
Every storefront has exactly one primary localization. In Germany it's German; in Brazil it's Portuguese (Brazil); in many smaller markets it's English (U.K.), which Apple uses as the fallback default. The primary localization is what shows to the majority of users browsing that storefront, so it carries the most weight for both conversion and keyword ranking — it should hold your strongest, highest-volume keywords.
Because the primary field is the one most people read, it's where brand name, the most important keyword in the title and subtitle, and the clearest value proposition belong. Secondary localizations are for extending coverage; the primary is for winning the terms that matter most in that market.
Example
For the France storefront, French is the primary localization, so the title and subtitle French users read live there — while English (U.K.) acts as an indexed secondary for extra keyword reach. Get the primary right first; it carries the most ranking weight and is what the majority of that market reads.