Hey ! I think you know it but the worlds created with pocketmine are very unrealistic and do not look like the worlds of Minecraft-Java. I tried to put a map Minecraft Java but the problem is that there are only generated chunks. So I want to know if there is a solution to this problem? (Plugin or other) Sorry for my bad english, I'm French.
Solution includes getting pwned by mojang for copyright. It's nearly impossible to recreate the perfect vanilla chunk generation, the softwares that have done it (like GlowstoneMC) use source code from Minecraft Java. According to Mojang's EULA, you aren't allowed to redistribute their code.
So if I understand correctly it is impossible to have a generation like Minecraft Java. But I know some servers that have a map that is very similar to Minecraft Java.
Launch Minecraft Java, go to the world, walk some chunks away (maybe 500 blocks from each direction of the spawn would be okay), go to server.properties and make the generator to be DEFAULT. Stop your server, copy the PC world to your server's worlds storage, start your server and tell me what happened. Make sure to set the world's spawn to the same coords as in Java edition, because PocketMine has those stuff a bit messed up.
If you search hard enough you can find similar questions. https://forums.pmmp.io/threads/trying-to-recreate-a-seed.5864/
There is no other way. Copy-paste the Java map generators to PocketMine's code or make your own generator my extending the GeneratorManager.