the new version of langprefix

drama woo!

Now to the actual thing:

where is the problem in just writing ex. {{etyl|proto:gem-pro|de}} or similar? I fail to see it. Or am I missing something here?

There is absolutely no problem with {{etyl|proto:gem-pro|de}}, either in theory or in practice. In fact, that already works; it's just that {{etyl|gem-pro|de}} also works.

But {{etyl}} is a special case, since (1) its argument isn't necessarily a language code, or even a fake language code — we can write {{etyl|Vulgar Latin|fr}}, for example — and (2) we can simultaneously have {{etyl:el}} and {{el}}, for example, if we want {{etyl|el}} to display "Modern Greek" while {{onym|el|foo}} links to [[foo#Greek]]. The etyl: templates really belong to {{etyl}}; probably we should have made them explicit subtemplates, by using names like {{etyl/Vulgar Latin}} instead of names like {{etyl:Vulgar Latin}}. The only connection between etyl: and proto:/cons:/etc. is that the former was apparently the inspiration for the latter.

A more typical case is {{term}}. Currently, something like {{term|...|lang=proto:gem-pro}} does not work. The problems are: (1) it doesn't know to link to an appendix (which can be fixed, by having it check for initial proto:); (2) it will actually put lang="proto:gem-pro" in the HTML (which would be easy to fix if we had Scribunto; since we don't, I think we'd have to create a new set of language subtemplates, called e.g. {{fr/HTML lang}} or whatnot, for mapping e.g. fr to fr and proto:gem-pro to x-gem-pro — though if we want, we could avoid creating it for mainspace languages . . . suffice it to say, if we want to go this route, I have some thoughts on the subject); (3) it uses the script-template {{Eror}} (because, for reasons I can no longer recall, {{Xyzy/script}} won't try e.g. {{fr/script}} if it finds that the language-code includes URI-special characters; that's easy to "fix", in theory, by removing that check, but I'm loath to do so without remembering why it was included to begin with); and (4) possibly other problems I haven't thought of. These problems are surmountable if we want to do it this way; personally, I think I'd prefer something like {{term|...|langprefix=proto|lang=gem-pro}}, but there are multiple valid approaches.