In Blood of Martyrs, the fluff describes a Sister Dialogous as an expert in languages, with an example of their prowess in Eldar. I made a Rank 8 NPC Sister to be an expert on a xeno-archeo site.
I found they don't get any langues beyond low, high and secret (admin), even at rank 8.
Correct me if I'm wrong, but you do Rank 1 from the Blood of Martyrs, pages 92 to 93. Then Ranks 2 - 8 from the Adept career. Which makes for a very cool Adept, one that can use Faith Talents.
But not useful if you want a language expert.
The fluff says these Sisters are known for speaking "new or unusual dialects" and skilled in "translating languages." It goes on to say they "specialize in languages..."
You could use elite advances to say they can learn to Speak any language 200xp each, or whatever cost seems appropriate for you campaign.
Or, you could use the Rogue Trader Into the Storm alternative rank Xenographer, but that seems an odd requirement for a Dark Heresy career.