get_default_locale() is broken

[espositofulvio]

Using thread locals for coroutines is not what you want

import asyncio
from threading import local
locals = local()
async def coro1():
   locals.locale = 1
   print(locals.locale)
   await asyncio.sleep(2)
   print(locals.locale)

async def coro2():
   locals.locale = 2
   print(locals.locale)
asyncio.get_event_loop().run_until_complete(asyncio.wait([coro1(), coro2()]))

[output]
1
2
2

If coro1 is executed before coro2, after the await it'll see modifications done by coro2 as they share the same thread. In the same way, a call to get_default_locale() after an await might return a different locale if another coroutine has changed it in the meantime. It's better to have users rely only on request.locale or you need to implement something like thread locals but for tasks.