flask-babel

Pass the locale selector function as the `locale_selector` argument to the `Babel` constructor or the `init_app` method. 
```python
from flask import Flask, request
from flask_babel import Babel

app = Flask(__name__)

def get_locale():
    return request.accept_languages.best_match(['en', 'de', 'fr'])

babel = Babel(app, locale_selector=get_locale)
# or if initializing later: babel.init_app(app, locale_selector=get_locale)
```

Ensure `Flask-Babel` is correctly installed in your current Python environment:
```bash
pip install Flask-Babel
```
And make sure to import from `flask_babel`:
```python
from flask_babel import Babel, gettext
```

1. Verify your `BABEL_TRANSLATION_DIRECTORIES` configuration in `app.config` points to the correct absolute or relative path of your `translations` folder. 
2. Ensure you've run the `pybabel` commands in the correct order: `pybabel extract`, `pybabel init`, and `pybabel compile`. 
3. Check that your `locale_selector` function (passed to `Babel(locale_selector=...)` or `init_app(locale_selector=...)`) is returning the desired locale string. 
4. Confirm that your translation directories (e.g., `translations/fr/LC_MESSAGES/messages.mo`) match the exact case of the locale codes your application expects, especially on Linux-based deployments. 
5. For strings outside the request context or in forms, consider using `lazy_gettext`.

Initialize the `Babel` object by passing your Flask application instance to its constructor or by using the `init_app` method:
```python
from flask import Flask
from flask_babel import Babel

app = Flask(__name__)
babel = Babel(app)  # Pass the app instance directly

# Or, if configuring later:
# babel = Babel()
# babel.init_app(app)
```