Most Japanese can read English and are very helpful. So, even if you cannot find a Japanese person speaking English, you can still write it down in beautiful hand writing (preferably in BOLD letters) and ask the nearest person available. The older generations do not necessarily know how to read because they studied Japanese when they were very young may be in school and its been decades since they ever used the language.Since English is taught in schools after the children are 12 years of age, it is recommended not to ask children either.
So, the age group to target while asking your doubts is 18-35 years. But most Japanese look atleast 5-8 years younger than their age, so better would be to target a person in the age group 22 – 40 years and you are most likely that the person will understand what you are asking (provided you wrote it down in simple and clean English). But, remember that they are very helpful and never say no to anything, so even if they are getting late for some meetings etc, they will stop to help you, so its recommended not to ask too many questions or too much of their time.
If you are lucky, you can find a Japanese who is an English speaker but chances are very low since only about 2% Japanese can speak English and most of them are located in the urban areas of Tokyo and around.
Another thing to keep in mind while asking any doubts is that you should ask only 1 question at a time, since Japanese people usually don’t like multi tasking and they prefer to focus on one thing at a time.
Also, remember to listen to them or do not ask another doubt when they are asking since this is considered very rude as it is considered a very bad practice if you obstruct someone in the middle of a conversation. So, just ask your question then wait for their response to get over, and when you think it is the right time to speak or write another question then only do so.