It is often said that (the) English taught in Japan is American English, but that is misleading. In fact, almost all English grammar books for Japanese high school students cover both American English grammar and British English grammar.
Is the mandatory or optional?
If it is optional, is there any difference in connotation between "English taught in Japan" and "the English taught in Japan"?
Sometimes, even with words that are usually non-count and general, we need to talk about specific "instances" of them.
For instance, "chocolate pudding" is usually just a general type of thing; we have "some" chocolate pudding, and don't use articles.* But a restaurant might say "Our chocolate pudding is made with Belgian chocolate," and mean by that "Our specific instance of the general thing." Similarly, you might say about that restaurant, "The chocolate pudding served there is made with Belgian chocolate."
Or, for an even more abstract example, a poet might write something about "the love I feel for you." Of course "love" isn't usually a "the," but they mean "the singular instance that I feel, out of the general experience of love."
* This is kind of a bad example, since we might say "I'll have a chocolate pudding" to mean "a serving of it," or "I'll have the chocolate pudding" to mean "the menu item that is chocolate pudding." Never mind these for now.
