Bot run to replace e.g. {{[[Template:l|l]]|ca|...}} with e.g. {{[[Template:l/ca|l/ca]]|...}} whenever possible.
List templates do, for example. I think there are others as well but I can't think of which.
As for being subtemplates, I don't think that is really such a big issue. And implementation details go both ways, too. A "l/ca" exposes no more implementation details than "l-ca" or some other variety. The first says "this is a subtemplate", the second says "this is not a subtemplate", which isn't that different. It would only count as an implementation detail if we asked users to understand subtemplates to use them, but right now an editor can easily treat "l/ca" as just a name like any other, so there is no problem. In fact, subpages in general are treated as just names by the software, and the subpage-ness is only significant when using things like {{SUBPAGENAME}} and through the links to the base page shown at the top of the page.
Furthermore, I believe that implementation details should not be hidden if they aid in understanding without being detrimental to it. In the past, list templates were hidden behind calls to {{list}}, so that {{list|en|days of the week}} was translated into {{list:days of the week/en}} internally. Daniel Carrero-esque syntactic sugar in other words, which contributed nothing...
Darn it, I was hoping you would be categorically opposed, and would view it as a straw man, and we could suss out why {{l/en}} is different. But I guess it's not different, and we're just on different sides of it. :-P