Flask-Babel working in development but not translating in production

I have got Flask-Babel set up and working correctly on my localhost development app (with two languages: en_GB and en_US).

However, when I try to switch languages on the production app, it does nothing. The get_locale() function is returning the correct language (and I can see it is being called multiple times on a page refresh, suggesting it is being called for each required translation), but no actual translations appear.

As I say, it is working perfectly in the development app and I can’t see any reason why the behaviour should be any different in production. Any ideas would be much appreciated.

The translation files (messages.mo and messages.po) are stored in translations/en_us/LC_MESSAGES.

config.py

LANGUAGES = ['en_GB', 'en_US']

babel.cfg

[jinja2: megaseatingplan/templates/**.html]
extensions=jinja2.ext.autoescape,jinja2.ext.with_

__init__.py

# Initialise flask-babel
babel = Babel(app)

@babel.localeselector
def get_locale():
    try:
        print("Language loaded from current_user: "+str(current_user.language))
        return current_user.language
    except:
        print("Language loaded from browser: "+str(request.accept_languages.best_match(app.config['LANGUAGES'])))
        return request.accept_languages.best_match(app.config['LANGUAGES'])

Function for manually choosing language

@app.route('/switch_language/<language>')
@login_required
def switch_language(language):
    current_user.language = language
    db.session.commit()

    track_event("User", "Switch language", label=language, value=0, userid=current_user.userid)

    if language == "en_GB":
        flash("Language switched to English (UK)")
    elif language == "en_US":
        flash("Language switched to English (US)")

    return redirect(url_for('home'))

EDIT:
Some extra info. I have confirmed that the BABEL_TRANSLATION_DIRECTORIES is pointing to the correct place and have also use list_translations() to confirm that it is finding my en_US translation file.