delete all keys except one in dictionary

Why don't you just create a new one?

lang = {'en': lang['en']}

Edit: Benchmark between mine and jimifiki's solution:

$ python -m timeit "lang = {'ar':'arabic', 'ur':'urdu','en':'english'}; en_value = lang['en']; lang.clear(); lang['en'] = en_value"
1000000 loops, best of 3: 0.369 usec per loop

$ python -m timeit "lang = {'ar':'arabic', 'ur':'urdu','en':'english'}; lang = {'en': lang['en']}"
1000000 loops, best of 3: 0.319 usec per loop

Edit 2: jimifiki's pointed out in the comments that my solution keeps the original object unchanged.

pop() it via for loop like this

[s.pop(k) for k in list(s.keys()) if k != 'en']

This is quite fast:

En_Value = lang['en']
lang.clear() 
lang['en'] = En_Value

Iterate over keys() instead:

for k in lang.keys():
    if k != 'en':
        del lang_name[k]

If you're using Python 3 I believe you need to use list(lang.keys()) instead.