Note: Because I think this was meant tongue-in-cheek (which by the way is not "tongue AND cheek!") I'll answer in the same spirit, intending no disrespect.

Ah, but then this fails fluency, because pronoun and antecedent are not parallel. If the subject ("one") is singular, then its corresponding pronoun must also be singular. Thus "they are fluent" is incorrect because the construction has switched from singular to plural.

In addition, if I am not mistaken, one should not pair "one" with third-person pronouns such as "his" or "her" even though they are singular. One should rather pair "one" with "one's." One does one's best, of course, but one must realize, if one followed every rule, one would probably puke one's guts up before one has finished writing.