Ruling on making du`a in English

Making du`a in English or in any other language is permissible outside of salah (prayer). But it is not permissible to say any part of the salah in any language other than Arabic, according to the majority of scholars. 

But if the Muslim can stick to Arabic in all cases, especially in acts of worship – and du`a is an act of worship – that is preferable and is better.