I guess it woudn't be too much confusing in this particular context, but it could easily be, look at these translations in french :
- "thousand" is "mille" (but in Wow every one uses 'k' for kilo-)
- "million" is "million"
- "billion" is "milliard" (as you said, no mob has that much HP yet)
An uppercase M (for Mega-) would be more adequate in french. Or go with the scientific notation : 10^6 ! ^^