YOU SAID:

I can speak perfectly fine.

INTO JAPANESE

私は完全にうまく話すことができます。

BACK INTO ENGLISH

I can speak perfectly well.

INTO JAPANESE

私は完全によく話すことができます。

BACK INTO ENGLISH

I can speak thoroughly.

INTO JAPANESE

私は徹底的に話すことができます。

BACK INTO ENGLISH

I can speak thoroughly.