Convert characters to Ascii
I would use a combination of things:


Twig's replace filter to convert ä to ae etc.
Twig's convert_encoding filter to eliminate any remaining non-ascii characters
Craft's kebab filter to do its thing


and of course wrap the thing up in a macro

{% macro ascii_kebab(s) %}
  {{ s
    | replace({
        'ä': 'ae'
      })
    | convert_encoding('ASCII', 'UTF-8')
    | kebab
  }}
{% endmacro %}