You have to ask, "Which language will they be able to use more later in life. It's one thing to just teach them a second language. It's another for them to be able to learn it, to use on practical levels.
I learned French in high school. It didin't do me a bit of good as I didn't keep up with it, and a couple of decades later, the one day I was in Paris, I could hardly speak anything worthwhile, except tell the taxi driver, "La Louvre, sil' vous plait!" to take me to the Lourvre (sp?) Museum.
Since I lived in NYC & Los Angeles for several years, it would have been more practical for me to have learned Spanish than French. It is something that I could use to speak to people here every day. Conversely, if I was ever planning to move to French Canada (NOT!) French is the better language to know.
By heritage, I grew up with an Asian language in my house, but, the dialect, it turns out, is not the popular one needed to converse with most Asians nowadays.
I also know sign language. BUT, sign language is NOT the same as American Sign Language. Believe it or not, sign language is not universal around the world. Different countries, different regions of the world, each have their own sign languages. Certain signs, phrases & idioms are not universal. And each culture & ethnicity is very proud & territorial of their own Sign Language, so it's not like they are ever going to give up their regional differences to homogenize into a universal "Sign Language."
When I run into people who speak American Sign Language (ASL) we might not be able to understand each other well because their structure, phrasing syntax of HOW & when they use certain signs, as well as the signs, themselves, that are different, throw us off.
American Sign language is not the same as Signed English, which is closer to how hearing people speak. ASL leaves out many of the words used in hearing English, & has some totally different signs & meanings and signed in a different order (phrasing) than either spoken or Signed English. Had I known ASL was so much harder to learn, I would have learned it first over Signed English. It made it that much harder & frustrating to try to learn ASL.