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/) 
    ]
});

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$/, /^\.\/(fr|en)$/)
    ]
});