webpack.mix.js

// ...

let webpack = require("webpack");
mix.webpackConfig({
    plugins: [
      // Choose the language you want to keep (Ex: "fr")
      new webpack.ContextReplacementPlugin(/moment[\/\\]locale$/, /fr/) 
    ]
});

Add this at the end of your webpack.mix.js. It will work only if you use momentJS (but it seems it's included every time in my projects, I don't know why btw)

Credit: http://stackoverflow.com/questions/25384360/how-to-prevent-moment-js-from-loading-locales-with-webpack

Is there a way allow more than one language?

For more than one language you can use regex. I haven't tested the code below, but it should work.

let webpack = require("webpack");
mix.webpackConfig({
    plugins: [
      // Choose the language you want to keep (Ex: "fr")
      new webpackContextReplacementPlugin(/moment[\\\/]locale$/, /^\.\/(fr|en)$/)
    ]
});