On http://test.musicbrainz.org/artist/9bffb20c-dd17-4895-9fd1-4e73e888d799/aliases you can see that it has allowed me to set two aliases to "English". The idea is to represent the artist's primary name in that locale, so there should only be one.

Apparently the unique key is defined on (name, artist, locale) rather than (artist, locale).