When PGV has been configured to support multiple languages, PGV won't use the preferred language set in the user's browser configuration, even when that preferred language is one of the "active" ones in PGV's language configuration.
The Browser has been configured to ask for pages in Dutch, when such pages are available and supported by the site. PGV has been configured to default to English. This is set in the GEDCOM configuration. When the Dutch user accesses the PGV site, he sees everything in English even though Dutch is permitted by the site. He should be seeing everything in Dutch.
PGV should only use the default language when none of the browser's configured languages are marked as "active" in PGV's language configuration. For example, the Browser says, "I want to see everything in Greek". PGV doesn't have Greek enabled, but the GEDCOM says to default to French. The Greek user should then see everything in French until he switches to another supported language.