However, all aircrafts in that company registered in another country "B".
To my knowledge in order to fly commercially and to be the legal PIC of the aircraft the pilot must hold the respective license in accordance with the aircraft's registration (e.g. N-registered aircraft = FAA license, or EASA license = EASA registered aircraft) or hold both licenses and thus, has to comply with the respective (or both) currency requirements. If an airline operates aircraft (and crew) of foreign registration they must comply with the respective regulations and currency requirements. But since most countries are ICAO members anyway (except the country of West-Sahara and a few remote islands in the pacific) pilot licenses are issued in accordance with ICAO Annex 01 FCL (Flight Crew Licensing) and the currency requirements are pretty much similar (e.g. 3 T/O and landings within the proceeding 90 days and 6 approaches within 6 month or 3 app. within 3 months which amounts to the same anyway).