Using a custom json encoder (lazy strings)

app.py

from flask import Flask
from flask.ext.babel import Babel
from utils import LazyAwareJSONEncoder


app = Flask(__name__, static_url_path='')
app.config.from_pyfile(app.root_path + '/config.cfg')

app.json_encoder = LazyAwareJSONEncoder

utils.py

from flask.json import JSONEncoder
from speaklater import is_lazy_string


class LazyAwareJSONEncoder(JSONEncoder):
    def default(self, o):
        if is_lazy_string(o):
            return unicode(o)
        return JSONEncoder.default(self, o)