They may have tried that in the past to no avail. The episode specifically mentions there was several failed attempts at first contact.
A guide only works if you understand the guide. Think of it like this, if I sent you all 1679 ones and zeros from the arecibo message to you, but you didn’t understand the concept of zero, you’d have a hard time understanding what the message meant, even if the message itself was self explanatory.
Their language isn’t just grammatically different, it is uses a fundamentally different structure, and the Tamarian brain might be structurally different as well, able to easily account for language when young. So there may not be much of a “base language” that the metaphors are built on so to speak.