Hi.

When you use translator in dev; WDT calculate Fallback, Defined Message
BUT
It do this, with the lastest locale used in page and not a locale use during translation

Exemple :
Locale FR :
{{ 'hi'|trans }} => Say 'Bonjour'
Change locale with $this->translator->setLocale('en');
{{ 'hi'|trans }} => Say 'Hi'
Change locale with $this->translator->setLocale('An Unknow language');
{{ 'hi'|trans }} => Say 'Hi' //Fallback
Change locale with $this->translator->setLocale('en');

WDT Render : You havent fallback message. because it's post analyse.

Usecase : mass mailing with different language