Bluff, Bluff, Bluff to the Right and Left

[Hecuba]

Hm. I'm actually not seeing Tongues on the beguiler spell list. (No tounges, either. )

However, he does have Detect Thoughts and Comprehend Languages, and between the two of those he might just squeak by. He has the Conceal Spellcasting skill trick, so he might put one over on the druid.

If you can find a way to understand Druidic (fairly easy with magic), they you simply need a reason not to speak it.

A good option here is for the NPC to simply pass as mute. Then you're only tripped up by writing.

I would suggest finding a some way to communicate without speaking.
My preference would be to have custom magic item made and learn Drow Sign language.

Continuous ring of Nystul's Magical Aura & Comprehend Languages is safely under 10k gold (what with the 1st level spells and all).

EDIT, given the limited item availability post:
If you can pass the Comprehend Languages without the Magical Aura, Comprehend Languages is actually on your list.