I have not been fortunate enough to play the original *Assassins Creed*, only *Assassins Creed II* and up, so I'm hoping this is answered in the original game somewhere.

In the so-called Ezio trilogy (*Assassins Creed II* and it's direct sequels, *Brotherhood* and *Revelations*), your player relives the memories of his distant ancestor Ezio Auditore, an Assassin that lived in Renaissance-era Italy. As you live through his memories, the Animus device automatically translates the things you hear yourself and others say from Italian to English. 

However, the translation process seems a bit odd. For example:

* Everyone speaks English with an Italian accent; if you're hearing machine-translated speech, why add in an accent?
* Sporadically, words are left untranslated for no apparent reason. Some of these words are vulgarities, which I can almost understand, but the Animus sometimes fails to translate words (like "assassin") that are routinely translated properly.
* Other languages are left completely untranslated, even if Ezio speaks them fluently (e.g. a small bit of French dialogue in *Brotherhood*)

Out of universe, this is almost certainly done to add a bit of atmosphere, but is there any in-universe reason why it works this way?