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.