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.

If you both want to translate conversations (Polish/English) at the same time, you will need two WT2 Edge units, each requiring its own headset. The device is designed in such a way that it requires two headsets for two-way recording and playback.

Unfortunately, I can't say anything about the other products. I have bought them online and offline so that it works even if there is no reception.