But then, what should the AI research if you're a custom race?
Universal Translators, obviously your linguistic structure is completely incomprehensible to them.
Ok, amendment to the original plan...
2 non-exclusive research trees, Universal Translators+ Xeno Relations and Xeno Languages+Cultures:
Language and culture techs are cheap, but must be researched Per Race (and have multiple levels of stackable bonuses to diplomacy with that particular race).
UT and Relations techs are VERY expensive, and don't provide nearly as much bonus as the language techs, but apply to ALL races.
The logic? Would you be more inclined to work with a race who took the time to understand YOUR language and culture, or with a race who whips out the Cool Techno Gizmo of the month?
Another thought, maybe make the UT a trade good, that way it's a gamble going down that line, will another race build the UT before you?
Perhaps give the race that builds the UT an influence bonus?
Maybe even have multiple levels of UT techs, which start off with a diplomacy penalty and end in a moderate diplomacy bonus (to represent the fact that the first UTs weren't 100% accurate).