[FunctionalTests] Make localization test more robust
[FunctionalTests] Make localization test more robust

This flag allows us to control the language instead of it getting picked
up from the system.

apple/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager