With my students, I've almost never come across a situation where I was unable to communicate things to my students in English. I didn't have to switch to Japanese. Repeating myself or speaking slower did the trick most of the time. Simple.
Knowing Japanese will, however, ease your job significantly, because students tend to add in a few Japanese words here and there (or a bit more...) They also mumble a lot in Japanese, so of course you can pick up a lot of valuable information just from listening to their monologues—アンケートは英語で何って言うんだっけ, et cetera. You'll save time if you understand Japanese.
In fact, understanding Japanese is enough. I never speak Japanese to my students unless it's a grandma whose English skills only consist of John Lennon's and Paul Anka's song titles. I've noticed that immersion has the greatest impact on students. I use single Japanese words, such as 名詞 (noun) and 動詞 (verb), to make grammatical explanations as clear as possible, but I keep the rest in plain English: "'Assume' is a doushi; but 'assumption' is a meishi." You don't really need Japanese if you know how to express things clearly and slowly.