enable automatically-generated "pseudolocale"

[cfm]

#6380 (comment):

Why are these omissions so difficult to catch during manual testing in the string-freeze process? At that point in the localization cycle, strings not (or incorrectly) marked for translation are indistinguishable from strings not yet translated.

We could solve this problem at least for human eyes by turning on Weblate's "pseudolocale generation":

Pseudolocales are useful to find strings that are not prepared for localization. This is done by altering all translatable source strings to make it easy to spot unaltered strings when running the application in the pseudolocale language.

Originally posted by @cfm in #6380 (comment)

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Testing

In freedomofpress/securedrop-i18n-sandbox from weblate-sandbox.securedrop.org:

1. Pseudolocale defined: https://weblate-sandbox.securedrop.org/projects/securedrop/securedrop/eo/
2. Pseudolocale generation configured
3. Pseudolocale can be added manually to securedrop/i18n.json
4. Under make dev, pseudolocale can be selected in Source and Journalist Interfaces

Production

In freedomofpress/securedrop-i18n from weblate.securedrop.org:

• Replicate above configuration: https://weblate.securedrop.org/projects/securedrop/securedrop/eo/
• Document use in QA: https://github.com/freedomofpress/securedrop/wiki/QA-Procedures#localization-qa
• Update enable automatically-generated "pseudolocale" securedrop-client#1598

[cfm]

I've configured the "pseudolocale generation" add-on at weblate-sandbox.securedrop.org with these settings per Weblate and Microsoft recommendations:

Setting	Value
Source strings	English
Fixed string prefix	[
Variable string prefix	!
Fixed string suffix	]
Variable string suffix	!
Variable part multiplier	0.1

For the Target language setting, I tried both Weblate's recommended en_XA and a custom "unknown" locale und_US. Even once added in securedrop/i18n.json, neither is usable by Babel because neither is present in the Unicode Common Locale Data Repository:

>>> import babel
>>> babel.Locale.parse('en_XA')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/user/securedrop/.venv/lib/python3.9/site-packages/babel/core.py", line 330, in parse
    raise UnknownLocaleError(input_id)
babel.core.UnknownLocaleError: unknown locale 'en_XA'
>>> babel.Locale.parse('und_US')
Locale('en', territory='US')  # collides with real en_US

We ran into a version of this problem in #6406, where under test we cheekily defined:

securedrop/securedrop/tests/test_i18n.py

Line 43 in 9c11950

NEVER_LOCALE = 'eo' # Esperanto

I'll have to think more about this.

[cfm]

Having tried en_devel in addition to en_XA and und_US, we'll have to use NEVER_LOCALE = 'eo' after all, at least until Babel supports custom locales (python-babel/babel#454). I've created eo in production Weblate, configured the "Pseudolocale Generation" add-on, and documented how to use eo in QA in https://github.com/freedomofpress/securedrop/wiki/QA-Procedures#localization-qa.