system
A few points. 91.126 is for “When approaching to land at an airport…”. The regs are silent regarding the direction of turns when taking off.
However, as a DPE that part’s easy. The private and commercial ACS’s Traffic Patterns task (skill element 2) requires the applicant demonstrate the ability to “Comply with recommended traffic pattern procedures.”, so the AIM’s 4-3-3 applies. Right turns on takeoff from a left-pattern airport are cause for disapproval of an application for the aforementioned ratings.
Finally, what is a “traffic pattern”? 14CFR 157.2 states “Traffic pattern means the traffic flow that is prescribed for aircraft landing or taking off from an airport, including departure and arrival procedures utilized within a 5-mile radius of the airport for ingress, egress, and noise abatement.”