By default, GIMP will look for a locale/ directory under your plug-in folder, and the gettext domain would be the plug-in name.
In other words, you should be good to go here. Just don’t implement set_i18n at all and GIMP should find your .mo file.
You would implement it and replace "gimp30-python" by something else if for instance you were using another localization domain. For instance, set to "plop" if you had a plop.mo file instead of SnowCover.mo.