So this caught my interest...
```
$ python3 pwb.py interwiki -wiktionary -simulate -hintsonly -family:wiktionary -lang:en
Missing main page name
...
```
It is using
```
mainpagename = site.mediawiki_message('mainpage')
...pywikibot.Page(site, mainpagename)...
```
With a bit of debugging added
```
$ python3 pwb.py interwiki -wiktionary -simulate -hintsonly
'{{ns:Project}}:Main Page' contains illegal char(s) '{'
Missing main page name
translate: getting default site wiktionary None
title translate hint results: set()
NOTE: Nothing left to do
```
https://en.wiktionary.org/w/index.php?title=MediaWiki:Mainpage&action=edit shows `{{ns:Project}}:Main Page`, which it tries to feed into Page&Link and they fail.
https://en.wiktionary.org/w/api.php?action=query&meta=siteinfo shows mainpage is `Wiktionary:Main Page`.