I am running Windows 7. With a Pure installation of Geoserver 2.12.1 (2.12.0 is the same).
Trying to upload local layer with cyrrilic name:
Pure installation of Geoserver 2.10.0 (2.11.4 is the same). The similar uploading:
First image is showing that geoserver after 2.12 version has troubles with cyrrilic layer's names. Second is showing that if version is under 2.12, problem is gone.
Same data_dir, layers files and pure installations with default settings.
-Dfile.encoding=UTF8
config is useless.