aakk9999 - 11:55 pm on Jan 8, 2013 (gmt 0)
You posted in December about a ranking drop for your english language pages in this thread: [webmasterworld.com...]
Have your rankings recovered? If they did, then perhaps the ranking problem was caused by these 50,000 URLs Google has "discovered" and now that they are returning 404, Google reports these 404 errors.
If you did fix your site so that 404 response is correctly returned (when it wasn't previously), then the message on increased number of 404 in WMT is a normal situation. You should briefly review these URLs, then declare them as "fixed" and if they are not linked from anywhere any more (e.g. they were result of some kind of error on the site/hosting) then they will not re-appear again.
It is always worth checking few of these URLs by clicking on a URL in the 404 list and then looking at the tab "Linked from" to see if Google gives you any indication where it found such URL. Sometimes the "Linked from" tab will be empty, but sometimes it will show you where it found URL that now returns 404. This should give you some clues.
If it shows a page on your own site as "Linked from" source and this page either does not exist or when checked, the URL that returns 404 is not there any more, it was probably a temporary glitch that has been fixed since.
In this case the only thing you can do is keep declaring these URLs as Fixed and ignore the 404 message in WMT - these will eventually go away as Google drops 404 pages.