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One can only be so unsensible, you know.

That line came from an Anime, so it's Japanese to English translation. The original Japanese line is something like "A lack of common sense is allowed only to a certain extent(= Just get some common sense!)", which is my translation anyway.

But here I was wondering how that line could be translated into the one at the title. I'm not questioning the translation quality, just not getting used to the sentences start with One, always. And the key point, I think, lies with the word Only, but I can't quite grasp the whole idea.

Could you be so nice and parse it step by step?

One might find it necessary to question the quality of the translation. 

Translation for anime has a constraint that doesn't exist in text-only and audio-only media.  The audio has to match the video.  The new words are paired with the same old mouth movements. 

That being said, the two sentences in question do share some common meaning. 

One can only be so unsensible, you know. 

The "you know" at the end of this statement can mean things like as you do know or as you should know or even which I want you to know.  It also can mean pretty much nothing at all, acting as a semantically empty interjection.  I think we can safely ignore it. 

This usage of "one" is something that used to be called the impersonal pronoun.  It represents a person, but which person is left unspecified.  It might indicate me.  It might indicate you.  It might indicate someone, anyone, or everyone. 

In this context, the word "unsensible" does mean something like lacking common sense.  To my native-reader's eye, it's an unusual, somewhat unnatural word choice.  In other contexts this word might mean imperceptible or unconscious.  Without your alternate translation, I wouldn't have known which sense of this word was intended.  Perhaps those syllables were chosen to match the animation, rather than to sound clear and natural. 

The combination of "only" and "so" carries the same meaning as only to a limited degree or only in a limited manner.  A statement like "it's so good" uses "so" as an intensifier.  A statement like "it's good, but it's only so good" uses "only so" as a limiter. 

One can only be so unsensible, you know. 

A person can only be foolish to a limited degree. 

A lack of common sense is allowed only to a certain extent. 

Here we have three different sentences that express pretty much the same sentiment.  The original translation might be only so good, but it is adequate. 

"One" is somewhat archaic english, but it just means "a person" or usually is used just as a more unpersonal version of "I"

"Only" is used as a limiter here, the literal translation "to a certain extent" is correct.

"unsensible" is just the negative of "sensible" or, having sense. Not a word I've ever seen used before.

"you know" is used to emphasize the sentence, and to include the other person in the conversation.

So another translation could be "I can only be stupid to a certain extent" but that is somewhat weird english and really depends on the character who is speaking.

For "One can only be so unsensible, you know." I'd imagine an older lady, probably rich and/or noble. A very formal speaker who tries to look detached.