Does this intentionally not start with a slash? Will translations be in any other directory than the top-level translations dir?

(and yes I know this also applies to the lines below, but those are there already ;-) )

Running code from inside the source tree like this isn't that nice. Can't it be an entry point defined in setup.py?

Can you make those "look at" targets links to (a webpage showing) their actual sources?

You can just use pathlib.Path(self.app_root) / 'translations' instead of calling joinpath()

No need to construct a list to pass it to languages.extend(), use a generator expression instead.

This inner function isn't called here, and it looks like it will just immediately be garbage collected. My guess is that somehow this is prevented by the @babel.localeselector annotation, but this should be made explicit.

Annotate the return type.

Why is it necessary to both return the locale *and* set it on g.locale? This should be included in the docstring, as it hints as how the entire translation system integrates with the rest of Pillar.

I18 is an incomplete name, it should be I18N, which is short for "internationalization".
Also flip the condition and return early, so

if not self.config['USE_I18N'}:
    return

That'll allow you to un-indent the remainder of this function.

Same as above, can be flipped to if not trpath: return, un-indenting the remainder of the function by another level.

The message isn't reflecting the code. trpath can exist as a file, symlink, or socket, and the message will tell you it doesn't exist at all.

Use f-strings for formatting, so f'Translation path {trpath} for extension {pillar_extension.name} does not exist'
Don't end exception messages in a period.

Why does this have to be a function? And why can it be relative? What is it relative to? Does it still work when the translation commands are *not* given from the top level directory of the project?

Use single quotes when possible.

I'm not too keen on using a dict for this. It makes it hard to figure out which keys there are, and half of the dict contents are hardcoded anyway, so could be module-level constants. The only two non-hardcoded values will in the end be turned into strings '.' and './translations'.

It seems unclear to me why a new dict has to be constructed and three functions called, just to get these hardcoded settings that won't change between invocations.

Don't pass the entire command as a string, this is very fragile and a potential security flaw. Use a list of individual arguments instead, and don't rely on str.split() to do this for you. For example, a slight change in get_application_folder() could result in an absolute path being used, which could contain spaces, breaking this command.

Don't use subprocess.call(), as it doesn't check the subprocess' return value. Use subprocess.run() instead, as this is [the recommended way to use the subprocess module](https://docs.python.org/3/library/subprocess.html#using-the-subprocess-module).

Instead of '{messages_pot}'.format(**babel_settings), just use babel_settings['messages_pot']. It's more readable, more explicit, and faster to execute.

This is the exact same invocation as from the function above, so put it into its own Python function.

Declare the return type

What does "For Pillar" mean?

Why do you need a vars() call here? Just use args.mode and args.languages directly.

See earlier comment, should be USE_I18N. If you really want to be a special snowflake, at least use USE_I19 ;-)
FYI, "i18n" just means "an 'i' and an 'n' with 18 letters in between".

This needs documentation about allowed values. For example, a locale can also include a territory, a codeset, and a modifier. Especially territories are important, because nl_NL is not the same language as nl_BE.

Returning "" is in violation of the declared return type.

Why use parents[1]? Does this always exist?

Returning None is in violation of the declared return type and the docstring.

Use super().run() instead.

Does this file format support comments? If so, add a description of what the file is for, and where the syntax of this file can be found.

You mean:
(1) https://docs.python.org/3/library/gettext.html
(2) http://babel.pocoo.org/en/latest/messages.html
or
(3) The internal file: pillar/__init__.py -> return _('The submitted data could not be validated.'), 422?

L6G S7G N1W E3Y D1Y T3K Y1U!

I mean whatever you mean with "look at the Python code". If you tell people to look at something, it's nice to let them know where they can find it.

You can drop the superfluous parentheses.

An if not with an else clause is hard to read. This would be simpler to parse mentally:

if self.config['USE_I18N']:
    g.locale = request.accept_languages.best_match(self.languages, self.default_locale)
else:
    g.locale = 'en_US'
return g.locale

Every access to g requires going through a LocalProxy object. It'll be faster to determine the locale and store it in a local variable, then assign that to g.locale and return the local variable:

if self.config['USE_I18N']:
    locale = request.accept_languages.best_match(self.languages, self.default_locale)
else:
    locale = 'en_US'
g.locale = locale
return locale

Since this happens on every request, performance is important.

Add a debug log entry that explains it's going to skip I18N stuff.

Add a debug log entry that explains this extension doesn't have a translations path

This can never happen, given how the translations_path property is written.

Logging isn't used, so better to remove the import.

Does this really return the pathlib module?

The use of create_messages_pot() seems to consistently be create; run other command; cleanup. If the cleanup should always run, even when the "other command" fails, it would be better to turn create_messages_pot() into a context manager using @contextlib.contextmanager.

It's clear that this is a list; available_modes would be a better name. Or even easier, just use the list literal directly at the choices=... argument, since it's used only once anyway.

Are different codesets supported by Pillar then? Don't we always assume UTF-8? And what do those modifiers do? If you include this text, it's important to know what will work and what won't.

Don't use f-strings in logging. Now the string will be formatted even before doing the self.log.debug() call, which might drop the formatted string whenever debug-level logging is disabled. Use percent formatting and pass format arguments as extra args to self.log.debug().

Keep your imports of Python's stdlib together, without newlines, ordered alphabetically.

What should happen when an exception is raised? Shouldn't that also clean up messages.pot? According to the docstring it is, but the code doesn't do this.

Maybe mention that all translations should be in UTF-8?