I think I've recently found the same issue when implementing my first multilanguage DNN 8 (I have made a lot of them with DNN 7 without problems).

The only way of getting language selector working is using aliases with language parameter (en-gb, es-es, en-au, fr-fr, etc). If you try to use any other kind of aliases, they won't work. You may test it at http://test.london.dotware.es/

I tested it with a new DNN 8 installation and with an DNN 8 upgraded from a DNN 7 functional one. On DNN 8 the language selector doesn't work (it won't show the respective language aliases, but always the alias in which you are.

I've logged the issue and I hope we get a quick answer, or we'll forced to keep using DNN 7 for our multilingual sites: https://dnntracker.atlassian.net/browse/DNN-9361

Best,
Francisco