Program.cs

using System;
using System.Globalization;

namespace CustomLinqExample;

public static class CustomLinqProvider
{
    public static CultureInfo[] Where(this CultureInfo[] cultures,
                                        Predicate<CultureInfo> filter)
    {
        return Array.FindAll(cultures, filter);
    }

    public static T[] Select<T>(this CultureInfo[] cultures,
                                Func<CultureInfo, T> map)
    {
        var result = new T[cultures.Length];
        for (int i = 0; i < cultures.Length; ++i)
        {
            result[i] = map(cultures[i]);
        }
        return result;
    }
}

class Program
{
    static void Main(string[] args)
    {
        var commaCultures =
            from culture in CultureInfo.GetCultures(CultureTypes.AllCultures)
            where culture.NumberFormat.NumberDecimalSeparator == ","
            select culture.Name;

        foreach (string cultureName in commaCultures)
        {
            Console.WriteLine(cultureName);
        }
    }
}