Most likely, there never was a complete list. In October 2014 Peter Constable wrote the following in an e-mail to the IETF-languages mailing list (emphasis mine):
While ISO 639-6 did get approved and published, the code table for 639-6 has never been made fully available in a usable manner. What data has been available has been looked at by lots of people ...
For German there are
https://atlas-alltagssprache.de for differences in the usage of contemporary German, aiming at regional colloquial German, not at the basic dialects
https://www.uni-marburg.de/de/fb09/dsa/projekte/digitaler-wenkeratlas-diwa Maps German dialects at the beginning of the 20th century
https://www.regionalsprache.de/ with links to other ...
The online Arabic-English dictionary that reminds me most of the site you mentioned is PONS. It isn't exclusively an Arabic-English dictionary. Other language pairings are offered as well. To give you a taste of what it can offer, click on the link below:
You may also like WordReference.com. Here's what it does with this same word: