The short answer
The most common cross-border failure is conversion definitions differing by market so data can't be compared. Standardize conversion definitions, attribution windows, and UTM rules before you start.
Unify the conversion definition first. Whether inquiry, booking, or purchase is the primary conversion, applying the same standard across markets is what makes comparison possible.
Standardize UTM conventions up front. Inconsistent naming of media, campaigns, and creative fragments your attribution data.
Prepare consent-based first-party data and server-side measurement for cookie limits. The goal is consistent standards and evidence-based decisions, not perfect tracking.