How does Yuuki talk like a boy? In Sword Art Offline 2 episode 7, Asuna comments that Yuuki not only is a rare tomboy female player but also talks like a boy.

Now, I watched the English dub and didn't pick that up so I guess it is something you'd only hear when you watch the sub with Japanese audio, but even in Sword Art Offline which was never dubbed (only subbed with Japanese audio), I didn't get an impression that Yuuki was talking like a boy.

So how is it that Yuuki talks like a boy?
In Japanese, there are multiple words for "I", the most common being watashi, atashi, boku, and ore (I'll transliterate to romaji thoughout). Most of these pronouns are definitively gendered, and it would be quite the social faux pas to use an improperly gendered pronoun. This video does an excellent job explaining the subject.

From S02E19 Zekken


  Yuuki: Boku wa kore dake, dakedo ne.
  
  Translation: I'll just use this, though.


In SAO, Yuuki uses boku.

Boku is generally a male pronoun. However, in anime (not so often in real life) excessively tomboy type girls will use boku in an attempt to sound masculine. This is something that is generally impossible to accurately translate into English in an acceptable way, so most official translations simply ignore the distinction, but most fansub groups will add a translator's note demarking this.