Hmm, interesting! What word would you have used for Cas instead of malaikat?

There isn't another word. Malaikat is correct.

My brain works like this: The lore of the SPN'verse, told in English, has Christianity as a default. Malay/Indonesian words of religion, in general, I've mentally defaulted to Islam (which is problematic, I know), so when I read this fic using those particular words, I am parsing Castiel as a completely different creature from SPN lore.

Also, what horusporus said: Dean's grumbling sounds even more awesome in Indonesian.