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
closesodoo/odoo#19165
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>