Does the choice of programming language affect paper acceptance in mechanical engineering?
The short answer is, no.  I've never experienced or heard of a reviewer caring about what language was used for code in a science or engineering paper.

In any case, I don't think Python is "unconventional" in 2015.  Here are some well-known and widely used codes that can be used for CFD with Python front-ends:


http://pyfr.org 
http://fenicsproject.org
http://github.com/clawpack/pyclaw


Note that all of these use Python in combination with lower-level languages for performance.

I'll also mention the educational course CFD Python.