Example of how to customise the language dropdown in Sitecore

Gallery Languages.xml

<?xml version="1.0" encoding="utf-8" ?> 
<!-- \Website\sitecore\shell\Applications\Content Manager\Galleries\Languages -->
<control xmlns:def="Definition" xmlns="http://schemas.sitecore.net/Visual-Studio-Intellisense" xmlns:shell="http://www.sitecore.net/shell">
  <Gallery.Languages>
    <Gallery>
      <!-- CodeBeside Type="Sitecore.Shell.Applications.ContentManager.Galleries.Languages.GalleryLanguagesForm,Sitecore.Client"/ -->

      <CodeBeside Type="LanguageDropDown.GalleryLanguagesForm,LanguageDropDown"/>      
      <Script>
        window.onload = function() {
          var activeLanguage = document.querySelector('.scMenuPanelItemSelected');
          activeLanguage.scrollIntoView(false);
        }
      </Script>
      

      <Stylesheet Key="GalleryLanguages">
        .scMenuPanelItem, .scMenuPanelItem_Hover, .scMenuPanelItemSelected_Hover, .scMenuPanelItemSelected {
        padding-left: 0;
        padding-right: 0;
        padding-top: 8px;
        padding-bottom: 8px;
        }
        .scGalleryGrip {
        position: absolute;
        bottom: 1px;
        left: 1px;
        right: 1px;
        height: 10px;
        }
        .scLanguagesGalleryMenu {
        overflow: hidden;
        vertical-align: top;
        border-bottom: 12px solid transparent;
        -moz-box-sizing: border-box;
        box-sizing: border-box;
        width: 100%;
        height: 100%;
        border-collapse: separate;
        }
        div#Languages img {
        display: none;
        }
      </Stylesheet>

      <Border Width="100%" Height="100%">

        <GalleryMenu ID="Options" Class="scLanguagesGalleryMenu">

          <MenuPanel Height="100%">
            <Scrollbox ID="Languages" Class="scScrollbox scFixSize scFixWidthInsideGallery" style="padding-top:0 !important;" Height="100%" Width="100%" />
          </MenuPanel>

        </GalleryMenu>

        <Gallery.Grip />

      </Border>

    </Gallery>
  </Gallery.Languages>
</control>

GalleryLanguagesForm.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Sitecore;
    using Sitecore.Configuration;
    using Sitecore.Data;
    using Sitecore.Data.Items;
    using Sitecore.Data.Managers;
    using Sitecore.Diagnostics;
    using Sitecore.Globalization;
    using Sitecore.Shell;
    using Sitecore.Shell.Applications.ContentManager.Galleries;
    using Sitecore.Web;
    using Sitecore.Web.UI.HtmlControls;
    using Sitecore.Web.UI.Sheer;
    using Sitecore.Web.UI.XmlControls;
    using System;
    using System.Globalization;

namespace LanguageDropDown
{
    // Decompiled with JetBrains decompiler
    // Type: Sitecore.Shell.Applications.ContentManager.Galleries.Languages.GalleryLanguagesForm
    // Assembly: Sitecore.Client, Version=7.0.0.0, Culture=neutral, PublicKeyToken=null
    // MVID: 1B65E6AB-6430-418B-8C39-D08CBC5C2C45
    // Assembly location: C:\inetpub\wwwroot\Sitecore8XDb\Website\bin\Sitecore.Client.dll



    /// <summary>
    /// Represents a gallery languages form.
    /// 
    /// </summary>
    public class GalleryLanguagesForm : GalleryForm
    {
        /// <summary/>
        protected GalleryMenu Options;
        /// <summary/>
        protected Scrollbox Languages;

        /// <summary>
        /// Handles the message.
        /// 
        /// </summary>
        /// <param name="message">The message.</param>
        public override void HandleMessage(Message message)
        {
            Assert.ArgumentNotNull((object)message, "message");
            if (message.Name == "event:click")
                return;
            this.Invoke(message, true);
        }

        /// <summary>
        /// Raises the load event.
        /// 
        /// </summary>
        /// <param name="e">The <see cref="T:System.EventArgs"/> instance containing the event data.</param>
        /// <remarks>
        /// This method notifies the server control that it should perform actions common to each HTTP
        ///             request for the page it is associated with, such as setting up a database query. At this
        ///             stage in the page lifecycle, server controls in the hierarchy are created and initialized,
        ///             view state is restored, and form controls reflect client-side data. Use the IsPostBack
        ///             property to determine whether the page is being loaded in response to a client postback,
        ///             or if it is being loaded and accessed for the first time.
        /// 
        /// </remarks>
        protected override void OnLoad(EventArgs e)
        {
            Assert.ArgumentNotNull((object)e, "e");
            base.OnLoad(e);
            if (Context.ClientPage.IsEvent)
                return;
            Item currentItem = GalleryLanguagesForm.GetCurrentItem();
            if (currentItem == null)
                return;
            using (new ThreadCultureSwitcher(Context.Language.CultureInfo))
            {
                foreach (Language language in currentItem.Languages)
                {
                    /// **********************************************************
                    /// PUT YOUR LOGIC IN HERE TO CUSTOMISE WHICH LANGUAGES ARE LISTED
                    /// **********************************************************
                    if (currentItem.Name.ToLowerInvariant().Contains("o") && language.Title != "English")
                    {
                        continue;
                    }


                    ID languageItemId = LanguageManager.GetLanguageItemId(language, currentItem.Database);
                    if (!ItemUtil.IsNull(languageItemId))
                    {
                        Item obj = currentItem.Database.GetItem(languageItemId);
                        if (obj == null || !obj.Access.CanRead() || obj.Appearance.Hidden && !UserOptions.View.ShowHiddenItems)
                            continue;
                    }
                    XmlControl xmlControl = ControlFactory.GetControl("Gallery.Languages.Option") as XmlControl;
                    Assert.IsNotNull((object)xmlControl, typeof(XmlControl));
                    Context.ClientPage.AddControl((System.Web.UI.Control)this.Languages, (System.Web.UI.Control)xmlControl);
                    Item obj1 = currentItem.Database.GetItem(currentItem.ID, language);
                    if (obj1 != null)
                    {
                        int length = obj1.Versions.GetVersionNumbers(false).Length;
                        string str1;
                        if (length != 1)
                            str1 = Translate.Text("{0} versions.", (object)length.ToString());
                        else
                            str1 = Translate.Text("1 version.");
                        string str2 = str1;
                        CultureInfo cultureInfo = language.CultureInfo;
                        xmlControl["Header"] = (object)(cultureInfo.DisplayName + " : " + cultureInfo.NativeName);
                        xmlControl["Description"] = (object)str2;
                        xmlControl["Click"] = (object)string.Format("item:load(id={0},language={1},version=0)", (object)currentItem.ID, (object)language);
                        xmlControl["ClassName"] = !language.Name.Equals(WebUtil.GetQueryString("la"), StringComparison.OrdinalIgnoreCase) ? (object)"scMenuPanelItem" : (object)"scMenuPanelItemSelected";
                    }
                }
            }
            Item obj2 = Sitecore.Client.CoreDatabase.GetItem("/sitecore/content/Applications/Content Editor/Menues/Languages");
            if (obj2 == null)
                return;
            this.Options.AddFromDataSource(obj2, string.Empty);
        }

        /// <summary>
        /// Gets the current item.
        /// 
        /// </summary>
        /// 
        /// <returns>
        /// The current item.
        /// </returns>
        private static Item GetCurrentItem()
        {
            string queryString1 = WebUtil.GetQueryString("db");
            string queryString2 = WebUtil.GetQueryString("id");
            Language language = Language.Parse(WebUtil.GetQueryString("la"));
            Sitecore.Data.Version version = Sitecore.Data.Version.Parse(WebUtil.GetQueryString("vs"));
            Database database = Factory.GetDatabase(queryString1);
            Assert.IsNotNull((object)database, queryString1);
            return database.GetItem(queryString2, language, version);
        }
    }
}