(Former Mozilla employee, here) I'm completely speculating, but it's probably a matter of not wanting to complicate iterating on the translation engine by intr

(Former Mozilla employee, here) I'm completely speculating, but it's probably a matter of not wanting to complicate iterating on the translation engine by introducing a bunch of cruft from the Firefox build system (which, though it uses GNU make under the hood, is very much bespoke and complicated). Since the translation engine is intended to run on a product that hosts WASM, they might as well just build to that.