[TYPO3] "Show content from this page instead" doesn't work for multi-language site

[tapio]

> yet and don't really know where to start looking next. How do I figure 
> out whether an extension uses XCLASSes? 

look at ext_localconf.php, which tells what XCLASSes has been used, for 
example:
$TYPO3_CONF_VARS[TYPO3_MODE]['XCLASS']['typo3/db_list.php']= 
t3lib_extMgm::extPath($_EXTKEY) . 'class.ux_db_list.php';

You can't know it beforehand if the author doesn't tell you that it uses 
  XCLASS-implementation(s).