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.


""" Format() will return the language-specific output for float values"""
        self.ensure_one()
        if percent[0] != '%':
            raise ValueError(_("format() must be given exactly one %char format specifier"))
        formatted = percent % value
        
        if grouping:
            lang_grouping, thousands_sep, decimal_point = self._data_get(monetary)
            eval_lang_grouping = safe_eval(lang_grouping)
            if percent[-1] in 'eEfFgG':
                parts = formatted.split('.')
                parts[0] = intersperse(parts[0], eval_lang_grouping, thousands_sep)[0]
                formatted = decimal_point.join(parts)
            elif percent[-1] in 'diu':
                formatted = intersperse(formatted, eval_lang_grouping, thousands_sep)[0]
        return formatted
def split(l, counts):
    """
    >>> split("hello world", [])
    ['hello world']
    >>> split("hello world", [1])
    ['h', 'ello world']
    >>> split("hello world", [2])
    ['he', 'llo world']
    >>> split("hello world", [2,3])
    ['he', 'llo', ' world']
    >>> split("hello world", [2,3,0])
    ['he', 'llo', ' wo', 'rld']
    >>> split("hello world", [2,-1,3])
    ['he', 'llo world']