According to the documentation and especially the Language Switcher, we're supposed to be able to do :

{{ url( location, {"siteaccess": ezpublish.translationSiteAccess( lang ) } ) }}

But there is "maybe" an issue when you are in and Match/URI if you don't set something for the default siteaccess.

Example:

    siteaccess:
        default_siteaccess: site
        list:
            - site
            - site_fr
            - site_de
            - administrer
        match:
            Map\URI:
                fr: site_fr
                de: site_de
                administrer: administrer
                manage: administrer

In this case I want the "/" without any URIElement to be the siteaccess "site"
But it doesn't work because the kernel doesn't find the matcher for the default siteaccess, I have the notice because we enter here :
https://github.com/ezsystems/ezpublish-kernel/blob/master/eZ/Publish/Core/MVC/Symfony/SiteAccess/Router.php#L241

I figured out a workaround :

        match:
            Map\URI:
                fr: site_fr
                de: site_de
                administrer: administrer
                manage: administrer
            Map\Host:
              myhostname.com: site

I'm not sure, how to fix this case.
Probably here : https://github.com/ezsystems/ezpublish-kernel/blob/master/eZ/Bundle/EzPublishCoreBundle/DependencyInjection/EzPublishCoreExtension.php#L88

I'm not event sure if that is an issue, but it was completely possible in eZ 4, maybe a tip in the documentation could be enough.

Otherwise, I did a PR with another workaround. I'll put it in comment.
Thank you