Topic on Extension talk:Translate

I found in Manual:$wgPageLanguageUseDB that we forgot the second line:

$wgPageLanguageUseDB = true;

$wgGroupPermissions['sysop']['pagelang'] = true;

I just saw in this topic below (https://www.mediawiki.org/wiki/Topic:Xlczwiqmyio6ymei) that it's possible to set different content languages for source pages,but I think I need further instructions.
So I have to add $wgPageLanguageUseDB = true; to LocalSettings.php, but how then do I mark the content language of a page?

And does that mean anyone can create a page in any language on my wiki? Is that a good idea, or are there downsides? We have 10 active languages and I guess the ability to create a page in a different language than English would encourage more people to create content.
The downside I see: If e.g. the page is created in Japanese than every translator needs to know Japanese to translate the page in any language, or can they still translate from the English translation somehow? Also information will have to be added in the Japanese version of the page first then, right? Or can I e.g. switch the content language without losing the text in any language?

There is a link in the sidebar to page information where you can find link to change the language. You can also go to Special:PageLanguage directly.

The correct language must be set before page is marked for translation and cannot be changed later.

There is currently no first class support for translating through a middle language, but translators can emulate that by ensuring that English appears in their assistant languages.

Hey, thank you, but somehow this doesn't work yet. We've added $wgPageLanguageUseDB = true; to LocalSettings.php but nothing happened. I can't access Special:PageLanguage or find the link in the page information page. Is there something else we need to configure first?

That is a MediaWiki core feature, so I am not familiar with debugging that. What version of MediaWiki are you using?