Impacted versions: 10.0

Steps to reproduce:
- Install a new language
- Open the language and click on "Deactivate"
![image](https://user-images.githubusercontent.com/6352350/33546866-7b2a9412-d8e2-11e7-818e-6a5be3801dc0.png)

You'll now get a traceback:
![image](https://user-images.githubusercontent.com/6352350/33546831-5ec2581e-d8e2-11e7-8887-08c0443bb1d8.png)

The issue is coming from https://github.com/odoo/odoo/blob/ab52c636c34b6f8b472a3b45ee162824de08fca4/odoo/addons/base/res/res_lang.py#L227


Current behavior:
Expected singleton error.

Expected behavior: Language is deactivated.


import json
import locale
import logging
import re
from operator import itemgetter
from odoo import api, fields, models, tools, _
from odoo.tools.safe_eval import safe_eval
from odoo.exceptions import UserError, ValidationError
_logger = logging.getLogger(__name__)
DEFAULT_DATE_FORMAT = '%m/%d/%Y'
DEFAULT_TIME_FORMAT = '%H:%M:%S'
class Lang(models.Model):
    _name = "res.lang"