Hi Jerome,
Let me give you as much information as seems somehow relevant to me:
The product listing menu item (ID 2621) is very basic as it lists all products in the shop. It's hidden, and serving only as menu item which all modules below category listings point to, and for the filter - this way providing a consistent URL.
All works perfectly fine (except for that darn "no results" message...):
The menu item's URL is "mydomain.xyz/shop", and if I enter the URL in the browser, it does indeed list all products.
As intended, all URLs of products are accordingly "mydomain.xyz/shop/........", including the ones found by the filter.
The filter module is also pointing to this menu item (2621), and as stated, it searches and finds correctly if the search term is good, or it shows no results if indeed nothing can be found. In any case, it returns the URL "mydomain.xyz/shop/product/listing", though - as mentioned - product links from search results are as expected "mydomain.xyz/shop/........".
Just for the jux of it, I just switched to Protostar, and also tried the language override on an almost identical site under J!2.5.28, HS2.3.5 and yet another template... same thing: all works fine, except no "no result" message anywhere.
I'm attaching screenshots, too:
1. The product listing config: Very basic with lots of "inherit" due to all default settings found to be good. Note that the associated category "Home" is the equivalent of the default "Product category" - the client had only renamed it. (Update: I reset it to "Product category", but no help.)
2. The filter settings: Again, working fine.
3. The filter module config: Note the menu ID, which happens to be the listing we're talking.
Please trust me: The "listing" view is untouched. Yes, I do know how to do language overrides - all others work, and no funky stuff in this one, only boring text in between the double-quotes. Of course, no cache anywhere near the site when trying.
Hope this info is sufficient. If you need anything else, like screenshots of the default config for the listing or so, please give me a brief hint.
Thanks again!