0.12.1dev: enable a default language to be configured in TracIni.
Setting is `[trac] default_language` and the values are the usual Locale keys (e.g. `fr`, `zh_CN`, etc.)
Factor out some of the logic in a `get_negotiated_locale` helper function placed in `trac.util.translation`.
Main part of #8117.
Authored by: cboos 2010-09-05
Parent: [r10051]
Child: [r10053]
