Here are the guidelines from American Airlines - it does address seats mfg outside of the US. I would check with your carrier to see if they have a similar page - then you can check the seat. fwiw, I can't imagine a new seat, especially a Britax not meeting requirements to be used on the aircraft.
HTH
again c&p from aa.com:
FAA Approved Infant Seat or Child Restraint Devices
Most restraints that are used in automobiles are acceptable for use in aircraft by an infant or small child. Acceptable restraints manufactured in the United States will bear one or both of the following labels:
"This child restraint system conforms to all applicable federal motor vehicle safety standards"
Additionally, the restraint may carry a second label with red lettering which states the following:
"This restraint is certified for use in motor vehicles and aircraft."
Restraints manufactured outside the United States may be accepted provided that they bear either a label showing approval of a non-U.S. government OR a label showing that the seat was manufactured under the standards of the United Nations (U.N.).
U.N. approval is designated by a label with a circle surrounding the letter "E" followed by the distinguishing number of the country which has granted approval, plus an indication of the category and mass group of the child restraint which will be affixed.
TJ