As said in this thread https://github.com/telerik/UI-For-UWP/issues/332 the decimal-separator is bugged when used in windows 10 1809, at least if the decimal-separator system-setting is other then a dot, e.g. in Germany which uses comma.
The Bug seems to be solved in the referred Branch by Nasko https://github.com/telerik/UI-For-UWP/tree/Nasko/update-sdk yet it is not available in NuGet, not even as a prerelease.
Also setting the decimal-separator manually is impossible, yet in 2016 it was said this change will be implemented https://www.telerik.com/forums/problem-with-decimal-separator
However, is there a schedule, when the Bugfix will arrive in NuGet? The recent version is 220.127.116.11 from October 17th 2018 while the fixed branch is from November 2nd 2018.