to answer this kind of question, you have to know the purpose of

the employer wants to know whether you know yourself well or not, and also wants to know whether you are sincere or not.

mostly, you should talk about the weakness that you really want the employer to know and also tell them you have some suggestions to avoid and prevent it.

for example, if i feel my english is not good enough,i will say: english is my second language, sometimes i might speak like an alian, so please feel free to tell me to say it again.

usually i don't talk about my speaking, but culture. i told my boss that "i am from different culture, sometimes i am still having culture shock, although i can understand your language, but i have to have a second to process how to react in the american way."  my boss laughed and said "of course you do!"