Research tells us that young children can become bilingual much easier than adults. With this in mind, research also tells us that two languages is enough for most young children to learn, at least until about the age of 12. Three languages too soon could easily become confusing. High school and college age youth who already have a functional use of two languages often pick up a third language quite well, and may even want to go further.
The Mother Tongue, is literally the language (and voice) that a mother speaks to a child. And usually, that is the main language throughout a childhood. If that language is spoken at home, it will probably always be the main language to that individual, and the language they think in.
In the United States, I recommend that if an English speaking family wants to introduce a second language in the home, that it be Spanish. The reason is two fold: Spanish has the same alphabet as English, and also, there will be opportunities to speak Spanish in the US. Most of the west, as well as Texas was part of Mexico before these territories were incorporated into the United States. Also, many people who have become naturalized American citizens in the United States are native Spanish Speakers. In many areas of the US there are Spanish speaking tv and radio stations.
Mexico, and South and Central America are our southern neighbors and are mainly Spanish speaking. How wonderful it would be if many English speakers in the US could also speak Spanish. Maybe then, much more respect and understanding could bloom and who knows what would happen. Understanding and peaceful cooperation? dh
Language program offerings for all levels are online. Some are free and some are tuition based. For starters try:
Books on learning Spanish:
Public Library near you.