i guess it's just a common sence...
"english" is a kind of international language that can be used to communicate in different part of the world...
i guess nobody would speak in japanese in elsewhere except japan (& of course japanese will speak japanese anyhow...)
i can speak italian, but my hubby never said "激死!太太唔教bb意大利文" (to me, it's quite stupid to say so)
the point i guess most mama has here is:
唔教bb日文,唔洗"激死"!!!