Yes, I tested it all and it seems you can only select a certain amount of language presets before the default is affected. Any language selected under "English" will auto default to upload language. The first in the alphabet is selected.
"Site language" has nothing to do with, as that just reflects the User-Interface language. "Site language" can not determine upload language because there are more "Preferred subtitle languages" than there are "Site language" options.
Is there any specific reason why many languages are needed?
If it's just for search purposes, then there is an alternative way to search that you can use.
Option 1:
On any subtitle page under "Interesting links", you can click on "All subtitles for this movie" which will show you all subs for that movie/series/episode in other languages.
Option 2:
From any search page, i.e.:
https://www.opensubtitles.org/en/search ... uageid-eng
If you change the "eng" at the end of the URL to "dut" and hit enter, it'll show all subs under Dutch:
https://www.opensubtitles.org/en/search ... uageid-dut
Those last three letters in the URL can be changed to any language code and it'll have the same searching affect for the language. Or change to "all" for all language results.
Sorry, but this isn't something that can be bettered on this site.
The new website, once live, will behave differently and may meet your needs better.
Hope that helps.