Does a Nandroid Backup contain Language files? I know that a Nandroid backup is a snapshot of a system image. I wish to know if this backup will also include language files if Backup was made from a multi-language ROM.

For Example:

I bought a Lenovo phone (S890) but it only came with Chinese and English, My friend has the same phone but his came with a Multi-language ROM. I thought since we both have the same phone we can try and install the same recovery software on both phones. I can then create a Nandroid backup on his phone and use that backup on my phone to get Multi-lang too.

Is my assumption here correct ?
As a Nandroid backup creates images of all Android related partitions of the device (not of the radio-firmware, which is not directly related to any OS), your assumptions are correct: it covers everything, i.e. all data belonging to the Android installation. So provided both of you use the exact same device (compare the specs, some devices have variants which could be crucial when involving different hardware components), the copy as described in the question should work.

To be on the safe side, and have a chance to revert back, it cannot hurt to create a Nandroid backup on the target device as well, though :)