I expect that up until the article that is referred to in the tag "continue llcontinue" it will show all langlinks for all pages.
However for example the article "Oplegslot" show no langlinks in the generated list.

Note that values for 'continue' and 'llcontinue' are intended to be opaque data, the format of which may change at any time, so you shouldn't be trying to see which page might be referred to in there. That said, at the current time for that particular query llcontinue does refer to a page, and the result is indeed giving you langlinks for all pages up to that page. The thing you're missing is that it does so in order by pageid: llcontinue specifies page ID 335463, while Oplegslot has page ID 4330222.

More generally, each prop module produces its results in some order, but that ordering is not part of the module's guarantees and might differ for different values of the module's parameters or for different versions of MediaWiki in order for the module to efficiently perform the necessary database queries.

When using generators and prop modules, you should take advantage of the 'batchcomplete' flag in the result as described at https://www.mediawiki.org/wiki/API:Query#batchcomplete to collect all the properties for each batch of pages from the generator, and only process the batch after having collected all the properties.