DetectLanguage is not always functioning properly, especially when the sentence is very short, (try e.g. the string "Hallo", which one would think to be quite ambiguous).
Also the function is not very well documented, especially as to what the result "und" means [ which I got when I tested =DetectLanguage("Παρασκευή 2 Οκτωβρίου 2009")]; however a test to discover which language is associated with the locale set for a spreadsheet does work:
=DetectLanguage(TEXT(NOW() , "EEEE d, MMMM yyyy G, QQQQ, hh:mm:ss a"))

This is an interesting development when users need to translate excel sheet items. This would not help people like me that own a website (Drupal, Wordpress, etc.) and need all domestic content automatically translated for visitors in order to leverage indexes and page ranking in search engines to boost my global traffic.

There are different solutions (I am very satisfied with www.nativetung.com)that accomplish this. Nonetheless, nice to see more attention being placed on going global by these companies.