You almost certainly want a new repository.
The purpose of the repository is:
- to track history and changes so you can compare them easily
- to manage branches and merges rather than just emailing patch files around and applying them to working directories manually
If you're totally rewriting a project from scratch then there is no point putting the rewrite in the same repository. You won't be able to apply patches written in the old language to your rewrite. Switching repos won't make the history in the old repo go away, andif you switch you won't have any weird interim stages where you have two languages kicking round in your repo.
The only reason I would even consider keeping the repository when changing languages would be if a) the languages are so similar that code can often be copy-pasted from one to the other without making any changes, or b) you have a project in which the majority of the functional content in version control is something like templates in a templating language you're retaining, and the language of the core which you're changing be translated line-for line into another language (and even then only if you know you're going to need to keep iterating the templates during the migration).