Avoid fatal errors in Special:AdvancedTranslate due to core json migration

Renamed %CODE% to CODE to fool the checker.

Exception from line 4059 of /www/translatewiki.net/w/languages/Language.php: Invalid language code "%CODE%"
*0 /www/translatewiki.net/w/languages/Language.php(4088): Language::getFileName('/www/translatew...', '%CODE%', '.php')
*1 /www/translatewiki.net/w/extensions/Translate/ffs/MediaWikiComplexMessages.php(605): Language::getMessagesFileName('%CODE%')

Change-Id: Ic1620c46c2003b01c4946bd82435fbd4b1f2ec09