I don't think you can do this with Mako, at least not in the way you want.
The % can appear anywhere on the line as long as no text precedes it;
One way you can do this is to not put the if-statement in the template. It seems you are trying to use a translation, so make the translation in the code you are using to feed the template:
text = 'Rappel de paimenet' if object.lang == 'fr_FR' else 'Payment reminder'
# Now your template doesn't need to use an if-statement
template = Template("""Status: {{text}}""").render(text=text)
Since it appears that you are using this for translations, I would look into using gettext and Babel. If you are using this in a web application, look into any translation-specific utilities (such as django-babel or flask-babel).