When I was learning my prayers, I told a scholar at al Azhar about my prayer cards and how I could pray the arabic aloud and recite the english silently in my heart. The scholar gave his ok. Of course, I can't remember who the scholar was, bah.

This would probably be a good question to ask scholars on different fatwa websites.

From my own limited reasoning and from what I know of prayer, it is often emphasized to think about what you are praying. For example, in fatiha, when you "Ihdinas siratal mustaqeem" show us the straight path, actually ask Allah (swt) this conciously in your mind. You're not just reciting words, you're asking the Creator to help you. Another example is when the imam is reciting part of the quran and it talks about hellfire, ask Allah (swt) to keep you from it, when the quran talks about Allah's mercy, ask Allah (swt) for His mercy, etc.

So, it would stand to reason that if you don't think in arabic, you have to think in english (or whatever your native language is). What better way to think about the words then to recite them in english in your heart? I would guess that reciting the english outloud would not be advisable, as our contemplation of the prayer is in our hearts and not on our tongues.