I started building my own custom language switcher.
I currently have a homepage with 'en' as primary and 'da' (danish) translated. I enabled the slug prefix under settings for the english primary and disabled "Enable Language Specific Redirects".
{% set currLang = content.language.languageTag %}
{{ currLang|pprint }}
{# here you set all the display names and language tags - you can modify, add/remove as you need - they have to be exactly! like en, en-gb, en-us...#}
{% set codes = {'English': 'en', 'Dansk': 'da'} %}
And then the loop:
{% for key, val in codes.items() %}
{% set URL = "/"~content.translated_content[val].slug || val %}
{{val|pprint}}
{{content.translated_content[val].slug || val|pprint}}
<li>
<a class="lang-option" href="{% if URL == '/' %}/{{ content.slug }} {% else %} {{ URL }} {% endif %} "><img class="lang-flag" src="FILE-URL-IN-THE-FILE-MANAGER/assets/flags/{{ val }}.svg" loading="lazy">{{ key }}</a>
</li>
{% endfor %}
However, as you might notice, I experienced that when i was on the english version the URL would return only "/" for the english link but "/da" for the danish version and vice versa. This was the case even when {{ val|pprint }} would actually return both "en" and "da" regardless of what pages i was on.
So i had to check if URL was only "/" and then output content.slug instead, to make both links in the language selector work regardless of what language i was currently viewing the page in.
But why is this necessary? Why is e.g. the danish language URL var returning only "/" when {{ val }} indeed does contain "da", on the danish version of the page?
