TYPO3 Core

Addition: I had to extend my quick fix a bit so that the XML sitemaps and root pages still work:

$slugCandidate = '/' . trim($candidate['slug'], '/') . ( strpos( $urlPath, '.xml' ) === false && $candidate['slug'] != '/' ? '/' : '' );

A crucial detail to add, why the fallback may not have worked either:
We do not use the language function with classical languages, but with branches. Specifically: there is no main language with its own domain, but only the branches (language > 0) have a domain and should be filled from the content of language 0 in general cases.

config.yaml

languages:
  -
    base: ''    
    languageId: 0
  -
    base: 'https://branch1.com'
    languageId: '1'
  -
    base: 'https://branch2.com'
    languageId: '2'

Basically, this is a configuration error.

1. a base should not be empty (neither main nor of a language. It should at least be `/` - but also kind of `unique`
2. if should end in `/` even if a full domain is given.

At least, the SiteConfiguration UI does not allow to add and save a empty base (for a language). That's mentioned in the label for the fields.

However, I will look into it - and eventually fix it. TBH - personally I would through a \Runtime exception for a wrong configuration.

However, you don't need to wait for bugfix on that and instead correct your configuration.

Thanks four your assessment.

I had also tried it with a slash - with the same inadequate result.
So a configuration error can be excluded here. So I can't correct this configuration either.

The desired setup is basically understandable?
I would very much appreciate it if TYPO3 could also map more complex issues and not just pure textbook cases.

A headless approach is also conceivable, in which no typical languages are used, but language 1 is web (web.brand.com), language 2 is app (app.brand.com) and so on. - and if a "language" is not filled, "language" 0 is used (to which no domain is assigned).

And in conclusion: If configured suffixes are not taken into account at all when routing (at least in the place I found), then that is probably a bug in my eyes. I really hope this can be fixed and I am confident if even I could solve this with a one-liner.

Thank you very much.