The underlying problem is that setenv is not thread safe as simultaneous call to getenv in multithreaded enviroment might lead to segmentation fault by attempting to dereference a null pointer (relevant glibc ticket).
Considering that snap is usually started in a separate thread, this creates a serious issue, because a wide variety of functions call getenv internally (see above ticket for more info).
I personally encountered this problem when using ekg (which uses snap internally).
To be fair, I'm not sure why locale is set there to begin with.