If I write a [[link]] it will be blue if the page exists and red otherwise. But if I write [[:sw:link]] that will be an external or cross-wiki link, that is never red, as if it were impossible to know whether that page existed in Swahili Wikipedia.
But determining the existence of a page is just a quick database table lookup, and all databases run on WMF's servers, so it shouldn't be more expensive to look up a cross-wiki link, as long as it is one of WMF's wikis.
In Wiktionary, it is common to link to entries in foreign languages both on the local wiki and to the native wiki for that language. For example, in English Wikitionary the entry for "blue" links to the Swahili word "bluu" both on en.wiktionary and on sw.wiktionary, using the template {{t+|sw|bluu}}.
https://en.wiktionary.org/wiki/blue#Translations
But since the Afrikaans translation "blou" doesn't have an entry on the Afrikaans Wiktionary, another template is used: {{t|af|blou}}. And it is a pain to know which one of these two templates to use. If it was possible in {{#ifexists}} to determine the existence of a page in another wiki, only one template would be needed, and the bot job to change to the right template would not be needed.
#ifexist already works across namespaces (well, of course), so is there any good reason it shouldn't work across wikis?
Oddly, the documentation says #ifexist is an "expensive" parser function. That doesn't make much sense to me. It's as if red/blue links were expensive, and most of our list pages should be banned. https://www.mediawiki.org/wiki/Help:Extension:ParserFunctions#.23ifexist