en_US may not be activated as it is possible to create a database in
another language using the database manager.

When trying to install a chart of account, the tax return entry tried
to format a date at the installation of the module, with no lang in
the context. The fallback was made on en_US but an error is raised if
that language is not activated.

As it is a very common scenario to retrieve a language from the
context, add a generic tool method to do it.

Replace and closes odoo/odoo#37629

closes odoo/odoo#37568

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>