Purpose: allow to check email preview in a specific language in order to see the result in a given translation context. Specifications * added preview_lang field to change language of template * set default language from the template by evaluating expression if lang field of template is set, This commit is related to task ID: 30245 closes odoo/odoo#19165 Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>