misleading error message when trying to create a site with non-latin characters

[alicewriteswrongs]

Steps to Reproduce

try a string like "这是我的“ or "русский язык"

Expected Behavior

If we disallow certain characters there should be an error that says something like "Sorry, you've got to stick to latin characters here"

Actual Behavior

it incorrectly says the issue is that the field isn't filled in.

Strangely, if you add some text in latin characters, so the title is somehting like "это мой website" or "这是我的website" then it seems to work alright, and the site name works correctly. Only the latin characters show up in the URL slug though:

[mbertrand]

There was a preliminary PR (#134) to deal with this but no final decision was made on whether non-latin characters should be allowed and the best way to handle them if yes, so it was never merged. The PR branch is very out of date now but still exists at mb/slugform