Confusing

1. The warning is translated, not just in English. But not in all languages. I translated a few ones for major languages only (assuming that translators that start to initiate translations in that language would understand it, but there are already over 60 languages, plus some fallbacks including from various existing or disabled variants). You may want to add other languages if you wish (or fix some translations, made from the original English, that you think are inaccurate or contains some typos, for example you may wish to fix the Chinese translations used in Taiwan, because you are concerned by this area).
2. the reasons are no given by the site admin, but what has to be done is explained in the link provided, which goes to a page giving some details than asking to visit the support if you want to enable a language (but be prepared to really work on elements that block this langauge to be enabled in the translation tool and talk with the admins). I took the sentence by collecting messages already given by administrators and copy-pasted across several pages, just to build a template with it. Multiple reasons were always given and displaying them all may give hints to translators about what they could be checked before enabling it.

So it's appropriate as is: it just informs that it is disabled, for any reason, and that translation is still not possible in the translation tool, that does not allow selecting it (and its the most important reason of this message : disabled (in the translation tool).

And there a good reason for doing this check: if we link directly to the translation tool with a language code not supported, the translation tool will still open but for translating another language (the current user's preferred language, which may not be the one intended, and they could then start overwriting existing correct translations for that other language by new translations for the disabled language).

We've already seen such overwrites which are undesirable and break existing correct translations by translations for another language: users just click and follow the link, and don't look precisely at top of page to see which language was effectively selected.

Note: some languages have been enabled in the past then disabled, and you can detect them by their statistic page or by looking at the list of their translators (which are not inactive, but whose list is still displayed). Languages that were never really enabled have no translators at all listed... But the links to the translate tool, and the statistics graphs are hidden for languages or variants that are disabled.

Regarding the last sentence of #2: That's the very issue. I don't think the template does, in fact, give any "hints to translators about what… could be checked"—or at least not sufficient "hints". Instead of just listing the different possibilities, it seems like you could link to pages that could be used to "diagnose" which reason is the appropriate one. Anyway, as you probably already know by now, I have invited other opinions about this.

Buyt you don't suggest any other link. For now only that link was given in the existing pages, from which there are several indications and an explicit request to contact the support.

Which "better" or additional link do you suggest in this message ? May be we can add a "reason=" parameter to the template, allowing to give additional or more precise hints ?

For the record:

1. I don't have any suggestions for better links because I am not very familiar with this wiki, so I would have to do more research to find good pages to link to. I was hoping other users who are more familiar with the wiki would have some good ideas.
2. I will not be adding or modifying any non-English messages in the template because I do not speak any language except English.
3. Yes, adding a reason= parameter does seem like a good idea.

And finally: I never said English was the only language provided by the template (something you claimed elsewhere). In fact, I used the phrase "the warning (in English, anyway)" here in order to explicitly allude to the fact that English was not the only language in the template (something I was anticipating you might mention in your reply) and to indicate that the English message was the only one I was talking about. I put "English" in parentheses in the phrase "the (English) message" on your talk page for exactly the same reason (meaning: the message is not only in English but that's the version I am talking about). Sorry that you misinterpreted this, but it's really beside the point, since any changes made to the English message could be replicated to the other languages in the template in the same way the original message was.