[ecesena]

I've been using 2 distinct languages for FE and BE from the very beginning of my project, and I think that's one the best decisions we made.

It is true, you have to duplicate your models i.e. physically write the same code in 2 languages, but with ORMs this is pretty fast. At later stages, the amount of dup code is always less and less. On the pros-side, this intrinsically prevents you from taking shortcuts, leading to better separation of duties.

It depends on how big you think your project will become, I guess. :)