[ecash-herald] consistently apply en-US locale for locale formatted strings

Summary: Can get mock generation inconsistencies if user without en-US locale settings generates mocks. Specify en-US everywhere.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16600