Inline wiki markup like [[:bn:|main page]] is not parsed. Most of the interwiki links (from Wikidata) are missing or wrong. If present as on the main page https://de.wikivoyage.org/wiki/Hauptseite then they are linked to the user namespace.
The prefix BN: has been set up as an alias for the Benutzer: namespace (per the config file). This overrides the interwiki prefix.
I'd suggest to seek community consensus to remove that namespace alias. To be honest, it's hard to see why an alias to the User namespace would be needed.
We should remove the namespace alias. It was never used. "user" can be used as a shorter alias, too. But the bn language code should work.
When we ask for the BN alias in the past nobody thought about Bengali Wikivoyage. But now it became reality.
Could you please post this to the local discussion venue in order to seek consensus? As you observe, I think it is unlikely that there will be objections, but we had better see what the community has to say. Configuration changes like this, small as they may seem, have proven problematic in the past when not supported by consensus.
Today I ask the community to get a consensus to remove the namespace alias bn:
The voting will take a few days.