Translation:Where is the restroom?
I'm from America and I resemble that remark! :P actually I put washroom too because, while otearai is a bathroom, it leterally means 'hand washing' and I don't feel that restroom is quite right as far as translations goes. I would use W/C too but that's ny British in-laws influence :3
I'm not a native speaker, but I've heard トイレ used more frequently in casual settings and お手洗い when you want to sound a bit more "proper".
I think your Japanese friend was amused that have to learn トイレ, because to them, it's 外来語 (gairaigo) or a word that came from a foreign language. So, for a foreigner to have to learn a foreign word, it seems nonsensical; I mean, you're foreign, don't you already know it?
If you feel your translation should be accepted, then you just need to report it. All accepted translations need to be added manually, so it's best to just stick to straightforward translations and report if the translation that came to your mind wasn't accepted (assuming it's correct of course) instead of wagging fingers at Duo for not accepting every synonym.
That's an excellent point. お手洗い would indeed be the best option in that situation, especially if you're not at a restaurant (which are likely to provide you with an おしぼり if you're in Japan).
Other words are far less natural:
- 流し台【ながしだい】("sink", "washstand") sounds archaic
- 洗面所【せんめんじょ】("washroom", "sink") sounds like your intention is to brush your teeth, wash your hair/face, do your make up or something.