Git encoding is determined automatically by GE and doesn't have to be set by user. Since msysgit-1.7.10 it is UTF8, before that version, it was equal to system default encoding. If you commited files with non ASCII characters using git version less than 1.7.10 (should not happen since minimal version is at least 2.0), your repository needs to be converted. Here is some help:
Since GitExtensions-2.34 GitExtensions encoding is no longer present. If you hadn't set it to UTF-8, you have to convert git global and local config files to UTF-8 without BOM.
The only encoding user can set is Files encoding - this is default encoding for files content. Better solution would be to determine encoding for every single file. This encoding could be set in .gitattributes file.