how to disallow fr in robots.txt file ?

I have blog with thousands of posts. And i tried to add plugin to autotranslate content in other languages but got many issues.

So my idea was to create another website with the same content and i add gtranslate for automatic translation and everything is OK at this stage. but... :

I disallowed french language in robots.txt file to avoid duplicate content using : Disaloww: /fr/

1. Is it bad for SEO ?

2. Is someone who tested this strategy ?