Hebrew is the name of a language, so it is uncountable and always takes a capital H: you speak Hebrew, you say something in Hebrew, and you never say a Hebrew or the Hebrew about the language. The same word names the writing system, so a sign can be in Hebrew whether you are thinking of the words or of the letters themselves. Because the letters run from right to left, a book printed in Hebrew opens at the side an English reader would call the back.

This sense is countable: one Hebrew, two Hebrews, the Hebrews as a whole people. It belongs to history and to old writing, so you meet it in lessons, in museums and in the Bible. When English speakers talk about Jewish people alive today, they normally say Jew or Jewish person instead. The people came first and the language took their name, which is why the same word does both jobs.