They are both correct except for the question mark at the end. Why English is important - this is a statement which could be the title of a book or essay where you then go on to explain "why English is important". The second one " Why is English important?" ....is asking a question.

"Why is English Important?" is the grammatically correct question. In English, Question words can be used both at the start of a sentence and at the start of a question, depending on word order. In the example you used, Why (Q. word) English (subject) is (verb) Important (adjective). This is a correct for of a sentence as it follows the grammatical English sentence order (Subject-Verb-Object) but in this example the sentence has no object. While the correct grammatical question form follows the order (verb-subject-object). Hope this helps.