烧瓶，jinja2：jinja2.exceptions.UndefinedError：'ngettext'未定义

如何解决烧瓶，jinja2：jinja2.exceptions.UndefinedError：'ngettext'未定义

我正在编写一个小烧瓶应用程序。这是我的python代码：

from flask import Flask,render_template
from jinja2 import Environment

from towngen import generate_settlement


app = Flask(__name__)
jinja_env = Environment(extensions=['jinja2.ext.i18n'])
app.jinja_options['extensions'].append('jinja2.ext.i18n')

@app.route("/")
def hello_world():
    return render_template("settlement.html",settlement=generate_settlement())

我使用的模板内部：

    {% trans count=settlement.features|length %}
    The settlement has {apnumber(count)} unique features:
    {% pluralize %}
    The settlement has {apnumber(count)} unique features:
    {% endtrans %}

这引发了异常：

Traceback (most recent call last):
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/flask/app.py",line 2464,in __call__
    return self.wsgi_app(environ,start_response)
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/flask/app.py",line 2450,in wsgi_app
    response = self.handle_exception(e)
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/flask/app.py",line 1867,in handle_exception
    reraise(exc_type,exc_value,tb)
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/flask/_compat.py",line 39,in reraise
    raise value
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/flask/app.py",line 2447,in wsgi_app
    response = self.full_dispatch_request()
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/flask/app.py",line 1952,in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/flask/app.py",line 1821,in handle_user_exception
    reraise(exc_type,line 1950,in full_dispatch_request
    rv = self.dispatch_request()
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/flask/app.py",line 1936,in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/Users/gru/Code/towngen/web.py",line 13,in hello_world
    return render_template("settlement.html",settlement=generate_settlement())
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/flask/templating.py",line 137,in render_template
    return _render(
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/flask/templating.py",line 120,in _render
    rv = template.render(context)
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/jinja2/environment.py",line 1090,in render
    self.environment.handle_exception()
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/jinja2/environment.py",line 832,in handle_exception
    reraise(*rewrite_traceback_stack(source=source))
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/jinja2/_compat.py",line 28,in reraise
    raise value.with_traceback(tb)
  File "/Users/gru/Code/towngen/templates/settlement.html",line 12,in top-level template code
    {% trans count=settlement.features|length %}
jinja2.exceptions.UndefinedError: 'ngettext' is undefined

我是否配置错误？我找不到能清楚描述如何添加该扩展名的文档。