[IMP] *: use res.lang methods

get_installed and _lang_get_id are both ormcached and correctly check
the context

Retrieving a res.lang from a code is a frequent action that can be
achieved with _lang_get (cf previous commit).
Using _lang_get ensure the active_test in the context is correct and
is not poluted with another context propagation issue.
odoo/odoo#35490 discussion is an example of bad context propagation

closes odoo/odoo#35504

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