- Fix multiple translations in a same process for different id_lang
If id_lang are mixed (1, 2, 3, 1, 4, 2...) the translations aren't correctly populated from the theme lang.php override if include_once is used.
include must be used in order to handle mixed id_lang in a same process.

Signed-off-by: hadjedjvincent <v.hadjedj@presta-module.com>
- Fix english translation for backoffice_order template

Signed-off-by: hadjedjvincent <v.hadjedj@presta-module.com>
Merge pull request #1044 from prestamodule/development

Mail translation methods fix & template translation fix