Upvoted because I want to see replies to this question.
My intuition is that language learning has other positive effects than just the practical one of allowing you to converse with other people in that language. For instance you say that math would be more rewarding in the long run. But why? Most people really don't have a practical need for math past arithmetic. But your intuition is that it is nonetheless rewarding. I strongly agree! But I think the same is true of language learning, for very similar reasons.
But I don't really know if my intuition is right. Maybe someone else here has a better answer.
I'm a native English speaker who learned French to a conversational level (~B2 in the European Language Framework) before I left my home country. That language skill helped me get a job where I am now. The country I am in is multilingual, so the ability to learn languages and a willingness to make an effort with other languages is very important.
I can however understand this attitude, especially if you don't travel enough to warrant it and don't really need to use it. I feel sad typing this, because I am personally very interested in languages, but language skills in my home country aren't particularly valuable or desired and English dominates. If you decide to dedicate that time to learning a different skill, well I can't really fault that.
However I'd make some observations. Firstly, knowing some words even if you travel infrequently can be helpful. Secondly, according to wikipedia, 12% of the US speak Spanish, 29% in California. That's not nothing.
As to the benefits, I personally think there are numerous. People say you "understand the culture" or something but I'm not sure you get this unless you immerse in a native-speaking country. What you get on a purely linguistic level is an understanding of grammar, an understanding of some word roots (especially if you learn French, which had a significant influence on English as a result of the Normal invasion, or Latin. Similar benefits probably exist if you learn other romance languages or maybe German/Dutch), and an understanding of how that language influences how the speakers approach the world. Also, if you ever want to help a non-native speaker improve their English, it very much helps to appreciate the struggles they're going through.
Since this is HN, I don't think my language skills have improved my programming at all. I don't like duolingo.
I do think differently in every language I know, even swearing mentally has distinct hue. It's a good question how useful is that, but there is no question languages add certain perspective.
My intuition is that language learning has other positive effects than just the practical one of allowing you to converse with other people in that language. For instance you say that math would be more rewarding in the long run. But why? Most people really don't have a practical need for math past arithmetic. But your intuition is that it is nonetheless rewarding. I strongly agree! But I think the same is true of language learning, for very similar reasons.
But I don't really know if my intuition is right. Maybe someone else here has a better answer.