Early Mac Systems were written in pascal, well, Clascal as they called their Object Pascal system in those days, Clascal was developed internally at Apple with Wirth as a consultant. Think Pascal was the primary alternative to MPW until mid System 6 era. MacApp remained Apple's primary API until the mid 90s, and it was 100% written in Object Pascal (formerly Clascal).

Some chunks of Windows were also written in Pascal, using Microsoft Pascal which they'd been using since their CP/M days.