[FIX] web: fix translation dialog for model terms

Before this commit, new model term translations cannot be saved if it has been
translated before.
The reason is while translating fr_FR
the backend needs the mapping {old_term_fr: new_term_fr} to update old_value_fr
However the UI sent {old_term_en: new_term_fr} which was wrong

This commit fixes the bug by changing the key of the mapping from the
old_term_en to the old_term_fr

opw-3074712

closes odoo/odoo#106174

Signed-off-by: Samuel Degueldre <sad@odoo.com>