The Japanese train is already running on a test track. The reason it takes so long to build is that they want to keep costs down to finance it privately. They could probably open it in '25 if they threw a lot more money into the project (which China will likely do).
I'd guess that they have the tech to build a maglev train. The German technology is mostly 20-30 years old and well known. They already have trains running, so figuring out how to build a train and track shouldn't be too hard.
Hmm, I assumed that part of the reason it would take so long is that it's mostly going to be tunnels through the mountains, and ultimately you can only bore so fast.
I'd guess that they have the tech to build a maglev train. The German technology is mostly 20-30 years old and well known. They already have trains running, so figuring out how to build a train and track shouldn't be too hard.