As stated by Honza in 2014 in https://bugs.gentoo.org/367017, if there is a SendEnv setting in the systemwide ssh_config file, there is no way to override this in the per-user config file. (https://bugzilla.mindrot.org/show_bug.cgi?id=1285) Setting SendEnv is NOT a good default for people that have non-english locales on their machines and ssh to machines that do not support this locale. Gentoo should not have a dubious global default configuration that cannot be overwritten. It is impracticable to assume all remote machines are under the users control and could be configured to support the non-english locale. Please also see the various problems reported in #367017 Therefore I believe #367017 should be reverted.
*** This bug has been marked as a duplicate of bug 367017 ***
I don't consider a revert request a duplicate. Please at least reopen the original issue for reconsideration or post a statement in either bug thread. My MacOS machine also sends unsupported LC variables and the cause of the problem is pretty much ungoogleable from the symptoms.
I have already made a statement in bug 367017. I'm not going to repeat myself every time somebody complains about it.
