How to work around the caching of wtforms validator message translations

app.py

from flask import Flask, request, jsonify
from flask.ext.wtf import Form, TextField, Email
from flask.ext.babel import get_translations, lazy_gettext as __
from flask.json import JSONEncoder
from speaklater import make_lazy_string, is_lazy_string

class CustomTranslations(object):
  def babel_gettext(self, s):
    return get_translations().ugettext(s)

  def babel_ngettext(self, singular, plural, n):
    return get_translations().ungettext(singular, plural, n)

  def gettext(self, s):
    return make_lazy_string(self.babel_gettext, s)

  def ngettext(self, singular, plural, n):
    return make_lazy_string(self.babel_ngettext, singular, plural, n)

class I8nForm(Form):
  def _get_translations(self):
    return CustomTranslations()

class ExampleForm(I8nForm):
  email = TextField(__('Email'), [Email()])

# idea from https://gist.github.com/pvanliefland/6009424
class LazyAwareJsonEncoder(JSONEncoder):
    def default(self, o):
        if is_lazy_string(o):
            return unicode(o)
        return super(LazyAwareJsonEncoder, self).default(o)

app = Flask(__name__)
app.json_encoder = LazyAwareJsonEncoder

@app.route('/submit', methods=['POST'])
def submit():
  form = ExampleForm(request.form)
  if form.validate_on_submit():
    return jsonify(result='success')
  else:
    return jsonify(result='failure', errors=form.errors)