Admin bar language issues

Look into late loading the interface l10n files before the admin bar loads; might need a nasty global `$l10n` reset or something.

Start by tracing the admin bar loading path and where the interface l10n files are loaded. Investigate whether the global $l10n state is involved before the admin bar renders. Done means the admin bar uses the correct language without requiring an unsafe reset.

Written by the indexing model from the issue text.