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.


"""Python's strftime supports only the format directives
               that are available on the platform's libc, so in order to
               be 100% cross-platform we map to the directives required by
               the C standard (1989 version), always available on platforms
               with a C standard implementation."""
            
            
            format = format.replace('%-', '%')
            for pattern, replacement in tools.DATETIME_FORMATS_MAP.iteritems():
                format = format.replace(pattern, replacement)
            return str(format)
        conv = locale.localeconv()
        lang_info = {
            'code': lang,
            'iso_code': iso_lang,
            'name': lang_name,
            'active': True,
            'translatable': True,
            'date_format' : fix_datetime_format(locale.nl_langinfo(locale.D_FMT)),
            'time_format' : fix_datetime_format(locale.nl_langinfo(locale.T_FMT)),
            'decimal_point' : fix_xa0(str(conv['decimal_point'])),
            'thousands_sep' : fix_xa0(str(conv['thousands_sep'])),
            'grouping' : str(conv.get('grouping', [])),
        }
        try:
            return self.create(lang_info).id
        finally:
            tools.resetlocale()
    @api.model
    def install_lang(self):