Before offering namaz saying the Niyat is Mustahab or a preferable deed. It is allowed to make Niyat in your heart without saying it. Niyat can be done in any language. There is no hard and fast rule that it should be done only in Arabic.
The niyat for namaz should be done in the following manner, “I pray two rakaat (or as many needed for that time) Fard (if its Fard, or say Sunnat if its Sunnat or say Wajib if its Wajib or say Nafil if its Nafil) for Allah Ta’ala, face towards Kabah - Allaho Akbar”.
If namaz is being offered in a mosque then one should add "Behind this Imam".
Keep Me In Your Prayers