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
"""
        This method is called from odoo/addons/base/base_data.xml to load
        some language and set it as the default for every partners. The
        language is set via tools.config by the RPC 'create' method on the
        'db' object. This is a fragile solution and something else should be
        found.
        """
        
        lang_code = (tools.config.get('load_language') or 'en_US').split(',')[0]
        lang = self.search([('code', '=', lang_code)])
        if not lang:
            self.load_lang(lang_code)
        IrValues = self.env['ir.values']
        default_value = IrValues.get_defaults('res.partner', condition=False)
        if not default_value:
            IrValues.set_default('res.partner', 'lang', lang_code, condition=False)
            
            partner = self.env.user.company_id.partner_id
            if not partner.lang:
                partner.write({'lang': lang_code})
        return True
    @tools.ormcache('code')
    def _lang_get_id(self, code):
        return (self.search([('code', '=', code)]) or
                self.search([('code', '=', 'en_US')]) or
                self.search([], limit=1)).id
    @api.model
    @api.returns('self', lambda value: value.id)
    def _lang_get(self, code):