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To expand a bit on what has already been said, you should find that Japanese customs and security staff, airport staff, hotel employees and indeed everyone who deals with the public in any capacity will be professional and polite. At the airports and in the main business/tourist/shopping areas of Tokyo, many people speak English; less so if you move out into the suburbs or visit one of the less touristy smaller cities.
I love traveling in Japan; their reputation for being polite and honest is, in my experience, well-deserved. I speak a very few words of Japanese but have had several enjoyable trips to Tokyo and beyond without any problems.
People who don't speak English will work very hard to communicate with you through gestures or sign language, or to find someone who can help you in English, and they will be apologetic that they can't do better. (I find this very humbling - I'm visiting their country and they're apologizing for not speaking my language.) Someone who speaks good English will often volunteer to help you if you look the least bit lost or confused (in my case, standing on the train platform looking at the schedule).
I can't say about breastmilk but Japan does enforce the same liquids rules - 1 quart baggie of 100 mL bottles - as the US. They also have a special scanner for liquids - I once had a 100 mL bottle taken away for this scan and then returned. If there is confusion you can expect them to be polite but probably firm in enforcing the rules.