「コンビニのトイレを借りたい場合、店員に一言断ってから利用しましょう。借りる際は何か購入するのがマナーとしては推奨されます」の英訳。When using the restroom, you should ask the staff before use. It is recommended as a courtesy to purchase something when using it.ですか？
You shouldn't use the bathroom at a convenience store without asking a store clerk first and you should buy something at the store as well. It's just good manners to do that in Japan.
この場合、次のような言い方ができますよ。
ーYou shouldn't use the bathroom at a convenience store without asking a store clerk first and you should buy something at the store as well. It's just good manners to do that in Japan.
「店員に声をかけずにコンビニのトイレを使わない方いいし、店で何かを買った方がいい。日本ではそうするのが良いマナーなんだよ」
ご参考まで！
カナダ 
