UPDATE: We solved it. See it below.

Same here with Odoo v9.

I realized that when I simply edit and save a template (without any modification) it becomes empty. There is still code in it but the mail preview (and the mail) is empty. I've tried it even by duplicating an original - same effect: the original works well, the duplicated one is empty. 

Very annoying since we would need to modify some templates. Did you find a solution since January?

It turned out that when I duplicate, or just open and save a template, it does not save the translation. And since we use Odoo in Hungarian language (like the original starter of the thread) the system tried to use the hungarian translation - which was empty. 

So be aware which language you use - or in our case which is your customer's language that you write to.