Microsoft just announced a new Windows Server 2019 Insider build 17709. The previous version was released a few days earlier which was build 17677. Just like office 2019 which is coming this year, Windows Server will also follow the same name-year contradiction, i.e., will be ready for general release later this year but will be called Windows Server 2019.
Build 17666 offers two new features such as Application compatibility and In-place OS Upgrade along with several other fixes.
Mind you that this build will expire December 14, 2018.
Windows Server 2019 preview is available in 18 languages and can be download in ISO format whereas in VHDX format English is the only language you get. For now, there is no need of product key during setup but the future pre-release builds will require the activation keys.
You can download Windows Insider Preview builds from here.