This is possibly one of the most useful phrases I have learnt in Japanese.
More useful than 今日は (konnichiwa) hello, ありがとうございます (arigatō gozaimasu) Thank you, and てつだって ください, (testudatte kudasai), Please help me. And much more useful than Watashi wa yubi no kankaku ga sukkari nakunatta (I have lost all the feeling in my fingers), which to this day I am still waiting for the right opportunity to use.
Being able to say "How do you say ..." followed by what you want to say, is not only very useful, but it goes a long way to starting and continuing a proper conversation in Japanese.
The simple phrase is nan to iimasu ka, however a full sentence would look like this,
Q: Nani nani (something) wa nihongo de nan to iimasu ka?
Q: Dog wa nihongo de nan to iimasu ka?
A: Inu iimasu.
Obviously this is more helpful if you are speaking to your sensei, or someone who knows a bit of English however, it can easily be modified when speaking only Japanese.
(pointing at unusual food item in restaurant)
Q: Sore wa nihongo de nan to iimasu ka?
A: Kore wa takoyaki (This is Octopus balls, no not literally, it's a delicious type of Japanese food).
Or even in this example:
(pointing at a sign you can’t read)
Q: Sain wa nihongo de nan to iimasu ka?
A: koko de tatte wa ikemasen.
Did you already know how to say "How do you say in Japanese"? Let us know how or when you have used it lately?