One only needs motivation to get started in learning any language.
If one moves into a new culture that (1) has an official national language different than one's own, (2) has a large number of dialects spoken in the same area where one is living and/or (3) encounters on a daily basis or regular basis still-another FL one might end up trying to decipher and/or learn three or more other languages at the same time, especially if one already has a desire or inclination to more intimately function in a new L3, L4, L5 etc.
In short, if one is not satisfied with English or some other adopted "universal language", one is motivated to learn more. In my own case, I am living in Oman and have brought rudimentary Arabic knowledge--i.e. with skills that I have yet to hone greatly. I am married to a Filipino now for 4 years whom I met while working in Kuwait. I am considering retiring in Philippines and in a region where several dialects are spoken. I am trying to listen to Filipino lyrics on mp3 player to and from work each day. Here in Oman I have students from two or three dialect speaking groups of unwritten nature--Jaballi and Mahri. I ask my students occasionally questions about these languages and compare them to Arabic. In short, motivation is prevalent for learning as much as I can about all languages--esp. as I live here and may be here for five or more years.
My daughter is now three and was born in the Philippines. She will want to learn the two dialects her mother speaks as well as Filipino. We primarily use English but she goes to a play school with Arabs and Indian students. My daughter watches Dora and other cartoons in English, Urdu, Arabic and sometimes French. Dora also teaches some Spanish--and I speak Spanish to my daughter as well. Across the hallway from us at our flat is an Arab family whom I tutor in German. (They desire to emigrate to Europe.) My daughter will hear me use the language if visiting that family with me. I also speak to her occasionally in German or count steps in German as we climb up them.
In 1983, I first moved to Europe and lived with a farming family in Alsace--on the border with Switzerland. When I arrived to work there for 6 months, I was asked whether I wanted to learn German or French. Only after 6 months had I come to know that the family normally spoke two German dialects in the house--both from Switzerland. the parents spoke the dialects and the children spoke French, dialects and slang. (When I went to work in Germany the next year, I could understand both high German and adapt quite well to dialects. )
In my experience of living, working in, & volunteering in 12 countries, I would say that it is not that uncommon for peoples to try and pick up multiple languages at the same time on Planet Earth. In Asia and Africa the case for doing so is obvious. It is becoming obvious for Europeans, but Americans are way beyond the curve on this.