I created a multilingual project using Flask with the help of Flask-Babel.
While the translation mechanism via GNU gettext itself is pretty straightforward,
I did not know how to inject the actual language into the Jinja template,
so I can use it to set <html lang="xxx">.
While it is odd that this is not mentioned in the documentation, once again StackOverflow offered solutions.
solution one
Just translate the language!
<html lang="{{ _('en') }}">
That was too simple to think of :-)
solution two
Inject it into the context.
babel = Babel(app)
...
@app.context_processor
def utility_processor():
return dict(lang=babel.locale_selector_func())
...
<html lang="{{ lang }}">