nlp.pipe(n_processes=x) does not work with doc._.language['language']

[donnydongchen]

Hello there,

I'm new to the spaCy universe. I've encountered this issue but it could be something I did wrong.

How to reproduce the behavior

import spacy
from spacy_langdetect import LanguageDetector

nlp = spacy.load('en_core_web_sm')
language_detector = LanguageDetector()
nlp.add_pipe(nlp.create_pipe('sentencizer'))
nlp.add_pipe(language_detector, name='language_detector', last=True)

text = ['I like bananas.', 'Do you like them?', 'No, I prefer wasabi.']
# works without 'n_process' 
for doc in nlp.pipe(text,             
                    disable=["parser"],
                    batch_size=250,
                    n_process=4  # keep taggern_process=2
                   ):
    print(doc._.language['language'])

Error

AttributeError: [E046] Can't retrieve unregistered extension attribute 'language'. Did you forget to call the `set_extension` method?

Environment

• spaCy version: 2.2.4
• Python version: 3.7.6

Possible Relevant Issues I found (non-extension)

explosion/spaCy#4903
explosion/spaCy#4737