Removed the UI language parameter from mkvinfo

On Ubuntu 13.04 I followed the installation guide on the website.

On running I got this error
```
$ ./xenonmkv.py ~/Desktop/example.mkv  
2013-08-29 21:06:23,539 - xenonmkv [CRITICAL] get_mkvinfo: Error occurred while obtaining MKV information for /home/billie/Desktop/example.mkv - please make sure the file exists, is readable and a valid MKV file  
```

The problem lies here:
```
$ mkvinfo --ui-language en example.mkv  
Error: There is no translation available for 'en'.  
```

A quick fix for this seems to be to remove the UI language - see attached pull request. Now I understand that this might cause problems for people with the language set to something non-english, so I suspect a deeper investigation might be required.