The Timekettle WT2 Edge only requires one offline language pack per headset, not per earbud, but if two people are using separate earbuds (each with their own device), then each headset must have the language pack installed. For example, if you want offline Polish ↔ English translation between two users, both devices need the Polish package. Offline language packs can be downloaded directly through the Timekettle app. As of the latest info, each pack costs around 50 CHF, and it’s a one-time purchase per language pair per device. You can install and activate it online via the app’s “Offline Packs” section. So in short: one pack per headset, both headsets need it if used independently.