Number formatting should be internationalized

A user tweet claimed that a lack of number localization made MetaMask very difficult for them to use: We use commas for thousands, their region used periods.

This should be derived from the user's region code.

Start by locating where the extension formats numbers and where the user's region code is available. Done means number formatting follows regional conventions, including comma-versus-period thousands separators, and the relevant behavior is covered by tests.

Written by the indexing model from the issue text.