On forums mentioned below there is content in two languages (both LTR): Dutch and English. The user community is divided based upon language in different categories. However, the forums primary language Dutch is used in every topic, even topic in English categories.
Example:
such as:
<html lang="nl" class="desktop-view not-mobile-device text-size-normal">
This puts Google on the wrong foot, since the content is actually English. Often, Google will ignore the incorrect language hint in the html-tag and try to establish the language itself. But this is not guaranteed to work in all cases.
It would be handy in terms of presenting relevant search results in Google to have the option for multi-lingual forums to optionally specify the primary language on each category and have all topics in that category return the chosen primary language and the default otherwise.