While you have been waiting for Windows 10 Redstone 4 update, Microsoft already started working on in Redstone 5. This Redstone 5 build on Buildfeed which was added to build lists few days back, has now been also spotted in manifest for language packs.
— BuildFeed (@buildfeed) February 6, 2018
This is the first build in the Windows 10 Redstone 5 version, and might not include many changes when compared to Redstone 4 last build. Microsoft is internally testing the first compiled version of Windows 10 Redstone 5. The language pack manifest file contains the build version as seen below:
This also suggests that Redstone 5 builds will start from 17600 onwards for the Insiders. Skip Ahead Insiders will be the first ones to get thier hands on it, while Insiders on Slow and Fast ring would have to wait a bit longer.