About [[Osm:Diary entries.diary entry.comment count/sc]]
That's an issue of this wiki that does not accept the "|zero=" parameter; you may still be able to translate without it, using only singular and plural (if the zero case is already one of them, it will be taken accordingly dor your language), but this means that you still cannot use special non-numeral forms for zero such as "No comment", which will then look like "0 comment" or "0 comments" depending on the language rules.
Is there no way to add "|zero=" to Sardinian too? I've seen that Italian, French, Spanish, Catalan, Portuguese, Venetian, Asturian, Occitan, Galician and many other languages all have it in their translations.
That's because they were first edited "without the zero option", then a requet to local admin (or to the upstream projet maintainer via bug requests) was made to have him adding the missing option (which the current version of the "Translate UI" extension still does not validate correctly.
There's a pending discussion about the problem of adding specific selectors to the PLURAL parser function. In almost all languages zero maps the same grammatically as the singular or plural plural type, but is distringuished only in lexical form or an alternate form (instead of using numeral figures), such as a grammatical or semantic negation like "no", "not", sometimes also agglutinated like "neither" or abbreviated with elision, but sometimes even if the 0 numeral is used this alters the grammar, for example of verbs, or the grammatical case. Those forms for zero are very language specific and contextual, it's hard to hard code in a generic plural form, and for some projects that have limited support of plural rules the number of allowed forms is fixed (including MediaWiki itself, not just GetText), and for now there's no clear support for fallbacks for additional alternatze forms, so they have to be tweaked one by one and tested.
So by default the "zero" plural form remains forbidden as there's still no way to automate how fallback work and how many forms are requried to validate the translation automatically: the "Translate UI" uses a conservative test using a static number of forms and, even if some projects support some additional forms with specific selectors. Note that even CDLR does no specify how missing plural rules are falling back: the only fallback rule supported for now is to fallback to the last plural form given, which is generally the generic plural (but this does not work well for languages where 0 is singular; and there are dificulties for Slavic and Celtic languages, as well as Arabic, or with specific terms that need specific forms, including in English such as "no one" instead of "no person" which are grammatically singular, or "zero persons" which is grammatically plural!). That's in fact still an issue also in CLDR, and there's no solution for now with GetText (so projects using GetText use specific code to handle the translation of zero values, using separate translation units). It's hard to formalize in rules that can pass an automated check and validation.