Current version of CS use Content-disposition filename header field to set correct filename during download.
This work correctly on IE 7/8 and not work on all other browsers.
This browser not UrlEncode filename field from Content-disposition field.
Simply change next httphandlers (web.config) to support national filenames.

<system.web>
	...
	<httpHandlers>
		...
		<add verb="GET" path="cfs-file.ashx" type="CustomCSHandlers.FileHttpHandler, CustomCSHandlers" />
		<add verb="GET" path="cfs-filesystemfile.ashx" type="CustomCSHandlers.FileSystemFileStorageHttpHandler, CustomCSHandlers" />
		...
	</httpHandlers>
	...
</system.web>

Create C# class library project CustomCSHandlers and add 2 files.

1.FileHttpHandler.cs

using System;
using System.Collections.Generic;
using System.Text;
using System.Web;
using CommunityServer.Components;

namespace CustomCSHandlers
{
	public class FileHttpHandler : IHttpHandler
	{
		// Requested URLS are in the format:  .../__key/filestorekey/path/filename

		public bool IsReusable
		{
			get { return true; }
		}

		public void ProcessRequest(HttpContext context)
		{
			CSContext.Current.AllowTokenRequests(false);

			ICentralizedFile file = CentralizedFileStorageProvider.GetCentralizedFileByUrl(context.Request.Url.ToString());
			if (file != null)
			{
				context.Response.Cache.SetAllowResponseInBrowserHistory(true);
				context.Response.Cache.SetValidUntilExpires(true);
				context.Response.Cache.SetExpires(DateTime.Now.AddDays(1));
				context.Response.Cache.SetLastModified(DateTime.Now);

				if (CentralizedFileStorageProvider.HasAccessValidator(file.FileStoreKey))
					context.Response.Cache.SetCacheability(HttpCacheability.Private);
				else
					context.Response.Cache.SetCacheability(HttpCacheability.Public);

				if (CentralizedFileStorageProvider.CurrentUserHasAccess(file.FileStoreKey, file.Path, file.FileName))
				{
					string path = file.GetDownloadUrl();
					int i;
					if ((i = path.LastIndexOf("/")) > 0)
					{
						string filename = Globals.UrlDecodeFileComponent(path.Substring(i + 1));
						path = path.Substring(0, i + 1) + "N$/" + filename;
					}
					context.Response.RedirectLocation = Globals.FullPath(path);
					context.Response.StatusCode = 301;
				}
				else
					context.Response.StatusCode = 403;
			}
			else
				context.Response.StatusCode = 404;
		}
	}
}

2. FileSystemFileStorageHttpHandler.cs

using System;
using System.Collections.Generic;
using System.Text;
using CommunityServer.Components;
using System.IO;
using System.Web;

namespace CustomCSHandlers
{
	public class FileSystemFileStorageHttpHandler : IHttpHandler
	{
		// Requested URLS are in the format:  ...__key/[filestorekey]/[path]/[filename]

		public bool IsReusable
		{
			get { return true; }
		}

		protected CommunityServer.Components.FileSystemFileStorageFile GetFileFromPath(string path)
		{
			int index = path.IndexOf("__key/");
			if (index < 0)
				return null;

			path = path.Substring(index + 6);

			index = path.IndexOf('/');
			if (index < 0)
				return null;

			string fileStoreKey = path.Substring(0, index);

			index = path.LastIndexOf('/');
			if (index < 0)
				return null;

			string fileName = path.Substring(index + 1);

			if (path.Length - (fileStoreKey.Length + fileName.Length + 2) <= 0)
				path = string.Empty;
			else
				path = path.Substring(fileStoreKey.Length + 1, path.Length - (fileStoreKey.Length + fileName.Length + 2));

			fileStoreKey = Globals.UrlDecodePathComponent(fileStoreKey);
			if (path.EndsWith("/N$"))
				path = path.Substring(0, path.Length - 3);
			else if (path == "N$")
				path = path.Substring(0, path.Length - 2);
			else
				fileName = Globals.UrlDecodeFileComponent(fileName);
			path = Globals.UrlDecodePathComponent(path);

			if (string.IsNullOrEmpty(fileStoreKey) || string.IsNullOrEmpty(fileName))
				return null;

			CommunityServer.Components.FileSystemFileStorageProvider fileProvider = CentralizedFileStorageProvider.Instance(fileStoreKey) as CommunityServer.Components.FileSystemFileStorageProvider;
			if (fileProvider != null)
				return fileProvider.GetFile(path, fileName) as CommunityServer.Components.FileSystemFileStorageFile;
			else
				return null;
		}

		public void ProcessRequest(HttpContext context)
		{
			CSContext.Current.AllowTokenRequests(false);
			CommunityServer.Components.FileSystemFileStorageFile file = GetFileFromPath(context.Request.Path);

			if (file == null)
			{
				context.Response.StatusCode = 404;
				return;
			}

			DateTime lastModified = (new FileInfo(file.FullLocalPath)).LastWriteTime;
			DateTime currentLastModifiedDate = DateTime.MinValue;

			#region If-Modified-Since

			DateTime.TryParse(context.Request.Headers["If-Modified-Since"] ?? "", out currentLastModifiedDate);
			if (Math.Abs(((TimeSpan)lastModified.ToUniversalTime().Subtract(currentLastModifiedDate.ToUniversalTime())).TotalSeconds) <= 1)
			{
				context.Response.StatusCode = 304;
				context.Response.Status = "304 Not Modified";
				return;
			}

			#endregion

			#region If-None-Match (ETag)

			long eTag = 0;
			if (long.TryParse(context.Request.Headers["If-None-Match"] ?? "", out eTag))
			{
				currentLastModifiedDate = new DateTime(eTag);
				if (lastModified == currentLastModifiedDate)
				{
					context.Response.StatusCode = 304;
					context.Response.Status = "304 Not Modified";
					return;
				}
			}

			#endregion

			if (!CentralizedFileStorageProvider.CurrentUserHasAccess(file.FileStoreKey, file.Path, file.FileName))
			{
				context.Response.StatusCode = 403;
				return;
			}

			context.Response.ContentType = CommunityServer.Configuration.MimeTypeConfiguration.GetMimeType(file.FileName);
			context.Response.Cache.SetAllowResponseInBrowserHistory(true);
			context.Response.Cache.SetLastModified(lastModified.ToUniversalTime());
			context.Response.Cache.SetETag(lastModified.Ticks.ToString());

			if (!CentralizedFileStorageProvider.HasAccessValidator(file.FileStoreKey))
				context.Response.Cache.SetCacheability(HttpCacheability.Public);
			else
				context.Response.Cache.SetCacheability(HttpCacheability.Private);

			string disposition;
			if (context.Response.ContentType == "application/pdf" || context.Response.ContentType == "application/octet-stream")
				disposition = "attachment";
			else
				disposition = "inline";

			if (context.Request.Browser.Browser.IndexOf("Netscape") != -1)
				context.Response.AddHeader("Content-disposition", disposition + "; filename*0*=" + context.Server.UrlEncode(file.FileName).Replace("+", "%20") + "");
			else
			{
				context.Response.AddHeader("Content-disposition", disposition); // filename=" + context.Server.UrlEncode(file.FileName).Replace("+", "%20") + "");
			}

			// Send files stored on UNC paths explicitly to avoid a bug with TransmitFile.
			if (!file.FullLocalPath.StartsWith("\\"))
				context.Response.TransmitFile(file.FullLocalPath);
			else
			{
				context.Response.AddHeader("Content-Length", file.ContentLength.ToString("0"));

				using (Stream s = new FileStream(file.FullLocalPath, FileMode.Open, FileAccess.Read))
				{
					context.Response.Buffer = false;
					context.Response.BufferOutput = false;
					try
					{
						byte[] buffer = new byte[64 * 1024];
						int read;
						while ((read = s.Read(buffer, 0, buffer.Length)) > 0)
						{
							if (!context.Response.IsClientConnected)
								break;

							context.Response.OutputStream.Write(buffer, 0, read);
							context.Response.OutputStream.Flush();
						}
					}
					catch (HttpException) { }
					catch (Exception ex)
					{
						new CSException(CSExceptionType.UnknownError, "FileSystemFileStorageFile", ex).Log();
					}
					context.Response.Flush();
					context.Response.Close();
					s.Close();
				}
			}
		}
	}
}

add CustomCSHandlers.dll to bin folder.