To me, an universal language needs to be rich. One example i can remember is about the word "Saudade", a Portuguese word. It roughly means "the 'i miss someone/something' feeling", so if you are with "saudade" of someone, then you miss that someone. Notice that, since "Saudade" is a feeling, you can use it alone, without mentioning who/what you miss, very much like saying "i'm furious.", or "i'm in love.". But in english, since this feeling is expressed by a non-intransitive verb, you must always specify the cause of the feeling, you can't say "i'm missing". Therefore, regarding this feeling, Portuguese is a superior language than English, as you can express it in more varied ways. Actually, people here in Brasil say that the word "Saudade" is exclusive to our language, but i'm not sure.
So, to me, a universal language should be the one who support the most expression of ideas, for only this way it would be able to sustain the cultural diversity in our world.
Oh, and introducing second languages does wipe out cultures. Considering a national scale, when you introduce a second language to a people, usually one of the two languages tends to fade, and its culture with it. See what happened with the Native Americans... The main problem with being bilingual is... why knowing two languages when i can know only one? For example, Portuguese people who came to Brasil absorbed the native languages that existed here, mostly the Tupi-Guarani.