I often shop at Chinese supermarkets. Almost EVERYTIME I go, everyone - employees and customers - speaks to me in Chinese! I understand the language a bit, but not enough to have a conversation.
Here is my list of suvival Chinese at Chinese supermarkets!
你好 (ni hao)
English = Hello
Greetings are very important! FYI: I have seen Chinese people greet each other "Hello," even on TV shows produced in China so it's your pick!
谢谢 (xie xie)
English = Thank You
One of the most important words in life, yet one of the hardest words to pronounce correctly! 
不客气 (bu ke qi)
English = You're welcome
When speaking in English, I have a habit of saying "No problem!" instead of "You're welcome" so I have a tendency to carry that habit when talking in Chinese as well.
对不起 (dui bu qi)
English = Sorry
I have been told by my Chinese speaking friends that depending on the situation, 对不起 (dui bu qi) may be too heavy or serious. Depending on the situation, it may be better to use "sorry" in English (because I am in the United States) . Maybe I can use 不好意思 (excuse me) if apporpriate?
没事 (mei shi)
English = it doesn't matter, it's nothing, that's all right, never mind
没（有）问题 (mei (you) wen ti)
English = no problem
At my level, I use 没事 and 没有问题 interchangeably.
用/不用 (yong / bu yong)
English = Need / No Need
I often use this when I am asked if I need to purchase a shopping bag at a cash register. However, there is a big "BUT." I often don't understand that question! 
It seems that they have so many ways to ask the question?!
有/没有 (you / mei you)
English = to have / don't have
When the store cashier asks me if I need to purchase a bag, I just point to my bag and say “有”(have). It's beautiful when we understand each other while keeping my speaking part to a minimum! 
可以吗？ (ke yi ma)
English = Can / May? (Is it possible?)
I often use this when I want to find out if the cash register is open, or if I can use a credit card to pay.
To English speakers, it sounds bit off to use this phrase to find out if the cash register is open. By saying 可以吗？, you are asking if you can use this register to pay. I have used this phrase many times so it works!
As for credit card use, I just point or hold and wave my credit card to the cashier and say 可以吗？. It works!
OK吗？(OK ma)
Yes, this is a perfect Chinese phrase! I heard in songs and on TV shows!
At my level, usage is basically the same as 可以吗？.
Of course, knowing your numbers is very important and useful.
I can also form a simple sentence like, "Where is ___?" However, I know I won't understand the answer so I don't dare to ask or start the conversation in Chinese!
Also, if people say something that I don't understand, I dare say, 对不起。我不懂中文。which means "Sorry, I don't understand Chinese." If I say I don't understand in Chinese, some may take it that I actually understand it!
But do you know what is most important?
SMILE！！！
Even when there is a barrier in communication, smiles will take you very far!
This is the article of same title written in Japanese.