まだ means "still" or "yet" in English. We can say "私はまだ日本語を勉強中です。" or "私は日本語をまだ勉強中です。"　Both are correct Japanese, and we don't care about the position of まだ so much. They means "I am still studying Japanese". 勉強"中(ちゅう)" represents "being" studying. We don't often use the kanji "未だ" even if it is a formal documents, because まだ is a little casual and light word. By the way, "私は日本語をまだお勉強です" is a little strange, because お勉強 shows studying itself, and it doesn't represent someone is studying. For example, if you say "(お)勉強は大変だ", it makes sense. It means "studying is hard( for me)".

私は（まだ）（日本語を）勉強中です 私は（まだ）（日本語を）勉強しています 私は（日本語を）（まだ）勉強しています／勉強中です