I think this will work – thank you ever so much for spending the time to answer this!
oops. just found the problem.
To test a mod_rewrite (and to make sure mod_rewrite was working), I created a folder called ‘es’.
As soon as I deleted this, things are back to normal.
I feel like a bit of an idiot now 🙂
I think this is closer to the answer, but it fails too:
RewriteRule ^/?categoria-producto([a-z/.]*)$ //es/product-category$1 [R=301,L]
