Don’t know, if this is a new (v5) issue or if I am just messed up, so I will ask here for following generations to find this. 
I have set up a multilingual site (kirby-5.0.0-rc.1) and I need to set 'url' => '/' in the local dev config (details here: N2S: Why does my domain-specific Kirby config not kick in? · Nils Mielke, the Website — see update at the top).
Also not wanting URLs of the primary language using its language code, I have put the same setting in site/languages/en.php.
This leads to site-internal page URLs being prefixed with // (e.g. //photography/trees) which leads to the protocol (https:) being prepended by the browser, thus making them unusable.
Likely this is due to the above correlation of double 'url' => '/' settings, as removing either one solves the page URL issue, but introduces either the problem described in my blog post or needs all languages to use their respective language codes in URLs.
Is there any way to mitigate for this?
Or is this a new (unexpected?) issue in v5?
(I have replicated this very setup with a fresh v5 rc1 Starterkit with the same result.)