builder.Services.AddResourceLocalization<LocalizationResource>();

builder.Services.AddResourceLocalization<LocalizationResource>(opt => opt.ResourcePath = "MyResources");

builder.Services.AddDbContextLocalization<MyDbContext>();

app.UseMonqLocalization();

app.UserMonqLocalization(new RequestLocalizationOptions()
{
    DefaultRequestCulture = new RequestCulture("en-US")
});

public class LocalizationResource : ILocalizationResource
{
}

builder.Services
    .AddControllers()
    // For data annotation attributes localization.
    .AddMonqDataAnnotationsLocalization<LocalizationResource>();

builder.Services.AddResourceLocalization<LocalizationResource>();

var app = builder.Build();

app.UseMonqLocalization();
app.MapControllers();

app.Run();

public class MyDbContext: LocalizationDbContext
{
    public DbSet<MyModel> MyModels => Set<MyModel>();

    public MyDbContext(DbContextOptions<MyDbContextOptions> options) : base(options)
    {
    }

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

        modelBuilder.ConfigureLocalizationContext(); // Configure localization tables.

        SeedData(modelBuilder);
    }

    static void SeedData(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Lang>(entity =>
            entity.HasData(new[]
            {
                new Lang { Id = "en-US" },
                new Lang { Id = "ru-RU" },
            }));
        modelBuilder.Entity<Resource>(entity =>
            entity.HasData(new[]
            {
                new Resource { LangId = "en-US", Key = "NameRequired", Value = "Name is required." },
                new Resource { LangId = "en-US", Key = "WrongId", Value = "Wrong Id." },
                new Resource { LangId = "en-US", Key = "default-name", Value = "Name by default" },
                new Resource { LangId = "ru-RU", Key = "NameRequired", Value = "Требуется указать название." },
                new Resource { LangId = "ru-RU", Key = "WrongId", Value = "Некорректное значение идентификатора." },
                new Resource { LangId = "ru-RU", Key = "default-name", Value = "Название по умолчанию" },
            }));
    }
}

builder.Services
    .AddControllers()
    // For data annotation attributes localization.
    .AddMonqDataAnnotationsLocalization<LocalizationResource>();

builder.Services.AddDbContext<MyDbContext>();
builder.Services.AddDbContextLocalization<MyDbContext>();

var app = builder.Build();

app.UseMonqLocalization();
app.MapControllers();

app.Run();

public class MyPostModel
{
    public long Id { get; set; }

    [Required(ErrorMessage = "NameRequired")]
    public string Name { get; set; }
}

public class MyModel
{
    public long Id { get; set; }

    [LocalizableProperty]
    public string Name { get; set; } = "default-name";
}

public class MyController : ControllerBase
{
    readonly IStringLocalizer _localizer;
    readonly ILocalizedModelBuilder _localizedModelBuilder;

    public MyController(
        IStringLocalizer localizer,
        ILocalizedModelBuilder localizedModelBuilder)
    {
        _localizer = localizer;
        _localizedModelBuilder = localizedModelBuilder;
    }

    [HttpPost]
    public ActionResult<MyModel> MyMethod(
        [FromBody] MyPostModel value)
    {
        if (value.Id < 1)
            return BadRequest(_localizer["WrongId"]);

        .
        .
        .

        var result = _localizedModelBuilder.Build(myModel);
        return Ok(result);
    }
}

[HttpPost]
public ActionResult<MyModel> MyMethod(
    [FromBody] MyPostModel value)
{
    if (value.Id < 1)
        return BadRequest(Resources.LocalizationResource.WrongId);

    .
    .
    .
}

public class MyTests
{
    public MyTests()
    {
        CultureInfo.CurrentCulture = new("en-US");
        CultureInfo.CurrentUICulture = new("en-US");
    }

    [Fact]
    public void MyTest()
    {
        .
        .
        .
    }
}