Is there an OCP-friendly way to dynamically choose which collection in a DbContext to access?

You can use TPH to simplify you context and action.

Models:

public enum PhraseType
{
    English,
    Spanish
}

public abstract class Phrase
{
    public int Id { get; set; }
    public string Text { get; set; }
    public PhraseType PhraseType { get; set; }
}

public class EnglishPhrase : Phrase {}

public class SpanishPhrase : Phrase {}

DbContext:

public DbSet<Phrase> Phrases { get; set; }

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    //...

    modelBuilder.Entity<Phrase>()
        .HasDiscriminator(p => p.PhraseType)
        .HasValue<EnglishPhrase>(PhraseType.English)
        .HasValue<SpanishPhrase>(PhraseType.Spanish);
}

Action method:

public IEnumerable<Phrase> GetPhrases(string language)
{
    // assuming the language parameter is "english" or "spanish"
    var theType = $"YourNamespace.{language}Phrase";

    //assuming your models are in the current assembly
    Type type = TypeInfo.GetType(theType, true, true);

    MethodInfo method = typeof(Queryable).GetMethod("OfType").MakeGenericMethod(type);

    var obj = appContext.Phrases.AsQueryable();

    var result = method.Invoke(obj, new[] { obj });
    return result as IEnumerable<Phrase>;
}

A little explanation:

You Use TPH (Table per Hierarchy) so you only need to create one DbSet.

When you want to return all EnglishPhrases you can use context.Phrases.OfType<EnglishPhrase>() but since your parameter determines the type, you need to use reflection to call the correct OfType method.

You can put all those reflections codes into a helper class so your action will be cleaner.

In future when you want to add more languages you just have to edit the PhraseType enum and add extra HasValue in your Fluent API. No changes are needed in the Action method.

I just tested this on my system and it worked.