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.


odoo/addons/base/res/res_lang.py
return self.browse(self._lang_get_id(code))
    @tools.ormcache('self.code', 'monetary')
    def _data_get(self, monetary=False):
        conv = locale.localeconv()
        thousands_sep = self.thousands_sep or conv[monetary and 'mon_thousands_sep' or 'thousands_sep']
        decimal_point = self.decimal_point
        grouping = self.grouping
        return grouping, thousands_sep, decimal_point
    @api.model
    @tools.ormcache()
    def get_available(self):
        """ Return the available languages as a list of (code, name) sorted by name. """
        langs = self.with_context(active_test=False).search([])
        return sorted([(lang.code, lang.name) for lang in langs], key=itemgetter(1))
    @api.model
    @tools.ormcache()
    def get_installed(self):
        """ Return the installed languages as a list of (code, name) sorted by name. """
        langs = self.with_context(active_test=True).search([])
        return sorted([(lang.code, lang.name) for lang in langs], key=itemgetter(1))
    @api.model
    def create(self, vals):
        self.clear_caches()
        return super(Lang, self).create(vals)
    @api.multi
    def write(self, vals):