Nested Marshmallow field does not show data in nested Schema

Solution
from flask import Flask, jsonify
from flask_sqlalchemy import SQLAlchemy
from flask_marshmallow import Marshmallow

app = Flask(__name__)
db = SQLAlchemy(app)
ma = Marshmallow(app)

class Language(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    code = db.Column(db.String(2), nullable=False)

    type_id = db.Column(db.Integer, db.ForeignKey('type.id'))
    type = db.relationship('Type', backref='languages')

class Type(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String, nullable=False)

class LanguageSchema(ma.SQLAlchemyAutoSchema):
    class Meta:
        model = Language

class TypeSchema(ma.SQLAlchemyAutoSchema):
    class Meta:
        model = Type
    languages = ma.Nested(LanguageSchema, only=('code',), many=True)

with app.app_context():
    db.drop_all()
    db.create_all()

    l = Language(code='en')
    t = Type(
        name='Grocery',
        languages=[l]
    )

    db.session.add(t)
    db.session.commit()

@app.route('/')
def index():
    types = Type.query.all()
    types_schema = TypeSchema(many=True)
    types_data = types_schema.dump(types)
    return jsonify(data=types_data)
{
  "data": [
    {
      "id": 1,
      "languages": [
        {
          "code": "en"
        }
      ],
      "name": "Grocery"
    }
  ]
}