tails-documentation broken for locales not translated in the Greeter

See e.g.:

$ LANG=fa_IR.utf-8 tails-documentation 
Traceback (most recent call last):
  File "/usr/local/bin/tails-documentation", line 102, in <module>
    helpwindow = DocumentationWindow(uri)
  File "/usr/lib/python3/dist-packages/tailsgreeter/gui.py", line 1112, in __init__
    TranslatableWindow.__init__(self, self)
  File "/usr/lib/python3/dist-packages/tailsgreeter/language.py", line 110, in __init__
    tailsgreeter.config.locales_path)       # NOQA: E128
  File "/usr/lib/python3.5/gettext.py", line 524, in translation
    raise OSError(ENOENT, 'No translation file found for domain', domain)
FileNotFoundError: [Errno 2] No translation file found for domain: 'tails-greeter'

(Note that we don’t have any translations for the Greeter for fa*.)

The failing code is:

self.translation = gettext.translation(tailsgreeter.__appname__,
        tailsgreeter.config.locales_path)       # NOQA: E128

The fix is to add fallback=True.

Related issues

• Blocks #13245 (closed)

Original created by @anonym on 15160 (Redmine)