I solved this problem with a CustomUrlSegmentProvider.

using Umbraco.Cms.Core.Models;

using Umbraco.Cms.Core.Strings;

namespace RoutingDocs.SegmentProviders;

public class CustomUrlSegmentProvider : IUrlSegmentProvider { private readonly IUrlSegmentProvider _provider;

public CustomUrlSegmentProvider(IShortStringHelper stringHelper)
{
    _provider = new DefaultUrlSegmentProvider(stringHelper);
}

public string? GetUrlSegment(IContentBase content, string? culture = null)
{
    var updatedName = content.Name?
     .Replace("ö", "o")
     .Replace("å", "a")
     .Replace("ä", "a")
     .Replace("Ö", "O")
     .Replace("Å", "A")
     .Replace("Ä", "A");

    var segment = _provider.GetUrlSegment(content, culture);

    return segment?.ToLowerInvariant();
}

}

using Umbraco.Cms.Core.Composing;

namespace RoutingDocs.SegmentProviders; public class CustomUrlSegmentComposer : IComposer { public void Compose(IUmbracoBuilder builder) { builder.UrlSegmentProviders().Insert

In Program.cs:

builder.Services.AddSingleton<IComposer, RoutingDocs.SegmentProviders.CustomUrlSegmentComposer>();

Remember to refresh the Umbraco-cache and re-publish all content.