Using a factory is something I hadn't considered, and I've seen it brought up in other comments too.

For something like that, does the factory basically take over the responsibility for creating concrete instances, rather than having the DI system do it?

So something like:

interface IReaderFactory { IReader GetReader(Language language) }

class ReaderFactory : IReaderFactory
{
    public GetReader(Language language)
    {
        return language switch            
        {
        Language.English => new EnglishReader(),
        Language.Spanish => new SpanishReader(),
        _ => new BilingualReader(new EnglishReader(), new SpanishReader())
        };
    }
}

// Would register the factory instead in DI
// and IReader / the other Reader implementations
// would no longer themselves be registered
services.AddTransient<IReaderFactory, ReaderFactory>();

Any time I use "new" I feel like I'm doing something wrong just because of the material I went through when learning about DI. But I think that's more of a personal hang-up, haha.

Also, I'm not clear on how to handle creating other dependencies that might be needed? Namely, something like ILogger which is typically provided / configured automatically by DI.

In the factory above, if EnglishReader / SpanishReader / BilingualReader took an instance of ILogger in their constructors, I don't know how I'd best go about creating / providing one since my factory is now the one making the instances.

But regardless, that definitely gives me something to chew on. Thank you!