As you know, Japanese people highly appreciate their language, even more than English as International language. Hence, when you're in Japan, if you say in English, "Do you speak Japanese?" do not confuse when they have "No" as the answer.
So I recommended you to learn some basic Japanese phrase before you travel to Japan. Here are some useful words and phrases:
Notes: Remember that Japanese pronunciation is more like Indonesian than English, example: "A" is pronounced as "A" not "Ei" as in English.
Yes = Hai
No = Iie (it sounds like: iiiie)
Thank you = Domo Arigato
Thank you very much = Arigato gozaimasu
You're welcome = Douitashimashite
Excuse me = sumimasen
Goodbye = Sayonara
Good morning = Ohaiyo gozaimasu
Good afternoon = Konichiwa
Good evening = Konbanwa
Good night = Oyasuminasai
I don't understand = Wakarimasen (read: wa ka ri ma seng)
Japanese = Nihongo (read: ni hong go)
I don't understand Japanese = Nihongo wakarimasen
How do you say this in English? = Kore wa Nihongo de nan to iimasuka?
Do you speak English = Eigo o hanashimasuka? (read: ei-go-o-ha-na-si-mas-ka?)
What is your name? = Anata no namae wa nan desuka?
Nice to meet you. = O-ai dekite ureshii desu.
How are you? = Ogenki des ka?
Onegai shimasu = please *
We can use this to order food, ask for help, etc. You can also use "kudasai" for ordering something. Example you want to order mineral water, you'll say "Mizu kudasai". "Kudasai" is used more in sentences.
Where is ... = ..., doko desuka?
For example you want to ask, "where is the toilet?". Toilet is "otearai". So you'll say: "Otearai wa doko desuka?"
What is this? = kore wa nani desuka?
How much is this? = kore wa ikura desuka?
For more phrases and words and how to pronounce them, please see this videos from Yuu Asakura, I think her video is very helpful to learn basic Japanese quickly :)
Have fun learning Japanese!