Improve translation handling for plain text templates
Follow-up on https://discuss.tryton.org/t/using-latex-with-tryton/3937/22:
Now it will be a good improvement for Tryton if the Report and Translation used the proper mechanism for plain text.
The current plain text templates doesn't use the gettext
translation mechanism to extract translatable messages, so the whole template is added to be translated.
Add the gettext
method so strings which have to be translated have to be explicitly 'marked' with _( <translatable string> )