14 CFR 121.571 defines the FAA's safety briefing requirement for scheduled air carriers. Passengers must be "orally briefed," but it doesn't say that passengers must understand the briefing.
AC 121-24C expands on the regulation and provides more information on what the FAA expects. It states:
The pretakeoff oral briefing should be given so that each passenger can clearly hear it and easily see required demonstrations.
But it does not say that all passengers must be able to understand it. It does, however, contemplate that language difficulties could arise when it comes to exit rows:
The information regarding exit seating
must be printed on the card in the languages in which briefings and oral commands are given by the crew
They recommend that exit row passengers receive individualized briefings, and most if not all US airlines require that you speak English (or another language used by the airline) to sit there, so that you're able to understand instructions in an emergency.
A post on the blog AirSafe News expands on this:
This advisory circular also requires that an airline provide passenger
briefing information in the languages used by the airline. However,
there are no requirements that every passenger should be given an oral
safety briefing in a language that is understood by that passenger
As a practical matter, many international airlines will provide the briefing in multiple languages (sometimes via subtitles on tv screens) to try to reach as many passengers as possible. If that fails, the safety information card provides much of the information in pictorial form. See our previous question In-flight safety instructions for deaf passengers.