What does the : www.site.com/en/our-clients/ url produce?
I will need a live link to troubleshoot it.

Yep, here is it.

Spanish default language, K2 cat items work it.mgscreativa.com.ar/mgsj3vm3/nuestros-clientes
English Lang K2 cat items fail it.mgscreativa.com.ar/mgsj3vm3/en/our-clients

Maybe you can't get into the category because of missing joomla! lang switcher cookie, so please, go to the homepage it.mgscreativa.com.ar/mgsj3vm3/ and try the menu item "Nuestros Clientes" and see how it works nicely, all K2 items go to detail flawlessly, but if you change the lang to english, then the "en/" prefix is added to the URL by joomla! (that's correct) and K2 item details fails, in that case, the menu item option is "Our Clients".

Both cats are different and contains items translated and assigned to the correct languages, also the menu items, point to the correct cats in english and spanish respectively...

PS: I have installed VirtueMart 3, so I can handle my store needs, and this error isn't present, I can access all my products details pages with no hassle.

Edit: Here is the screenshot of K2 Advanced SEF parameters i.imgur.com/sER0Zx2.jpg?1

Hello,

The link you sent me is not accessible.

Really sorry, please check it out again!

Spanish default language, K2 cat items work it.mgscreativa.com.ar/mgsj3vm3/nuestros-clientes
English Lang K2 cat items fail it.mgscreativa.com.ar/mgsj3vm3/en/our-clients

I think that K2 get confussed because the two cats have articles that title is the same in english and spanish, because are personal names that can not be translated, all titles repesent brands. In cat it.mgscreativa.com.ar/mgsj3vm3/en/our-clients, the only item that works is the one called
Mundo Insumos1...

Both links fail on my side.
They both redirect to /en/ which is the normal behaviour. Since there are two items with identical aliases, K2 has no way of knowing which item is which.

You can either reinstate the item's id, choose a different alias or map that item into a menu item.

Great, changed alias of items in spanish and english and worked

Nice to hear that the issue is resolved.