If you go into the ladies’ loos after lunch in this building, it’s full of Japanese ladies cleaning their teeth.

Actually it’s bad to brush right after a meal. The sugar from whatever you just ate weakens your enamel.

Should I run back and warn them?

Anyone know how to say “Stop!” in Japanese?Or do I just yell “HAAAAAI-YA!” and karate-chop them?

“sumi massen” (phonetic, no idea on the actual spelling) is some kind of “excuse me please”

You might want to lead with that

And then the karate chop action

But most importantly, how do you say “Brushing your teeth immediately after meals is bad for you because it strips the enamel?”

それはエナメル質を取り除くため食事の直後に歯をブラッシング悪いあなたのためです。

It is bad for you brushing your teeth after eating to remove the enamel

Maybe that’s too complicated.

How about “Drop the toothbrush or you’ll never see your children again”?

歯ブラシを落とせばまたは決してあなたの子供をもう一度見ない

which translates to: Never see your child again or you lose a tooth