Tor Browser sends users to tb-manual in languages that shouldn't

This happens only in Tails. Tor Browser in Debian forwards this users to the English version, but in Tails:

Steps to reproduce:

- start Tails in Danish

- open TorBrowser
- Click on Hamburguer menu - Help - Tor User manual

Error:
Tor Browser opens to https://tb-manual.torproject.org/da/ , which gives 404

What should happen:

Tor Browser opens https://tb-manual.torproject.org/en_US because there is no Danish translation.

It seems that we are overriding some Tor Browser script.

A parallel ticket was opened at https://trac.torproject.org/projects/tor/ticket/27613#ticket but I think we should have one.

Original created by @emmapeel on 15937 (Redmine)