Thanks to an awesome community K-9 Mail currently ships with support for 47 languages. The next beta release will add a Malayalam translation and increase this number to 48.
If you want to join this effort, head over to https://www.transifex.com/k-9/k9mail
@k9mail Thank you K9Mail for a active development team.. Since, it contains only around 975 strings, I encourage all translators to localizing this awesome app to your language...