MobileJawi is a keyboard app that allows you to type in Bahasa Malaysia using the Rumi text or Jawi text.
It has prediction capability and provide suggestions on both the scripts.
You can type "Saya" and see "ساي" appear in Jawi. Alternatively, you can type "ساي" and see "saya" appear in Rumi.
It also includes English keyboard if you want to write a message in that language.
Prediction is available in all the keyboards.