Simple translations with parametrized strings and infinite plural forms

t9n.js

/* Do not use code you have not read first.
 * Original file: https://gist.github.com/clamiax/ee6eea3010021bc62d188739b42c86cd */
let T9N_DEFLANG = null;

function _() {
	if(!T9N_DEFLANG)
		return "";
	return t9n_format(T9N_DEFLANG, ...Array.from(arguments)); 
}

function t9n_use(lang) {
	T9N_DEFLANG = lang;
}

function t9n_format(lang, label, ...argv) {
	const node = lang.catalog[label];
	const idx = lang.getpidx(argv[0]) || 0;
	const fmt = typeof node == "object" && node[idx] ? node[idx] : node;

	if(!fmt)
	    return label;
	return t9n_printf(fmt, ...argv);
}

function t9n_printf(fmt, ...argv) {
	let ret = "", len, i, ai;

	for(i = 0, len = fmt.length; i < len; ++i) {
		if(fmt[i] == '%') {
			++i;
			if(fmt[i] != '%') {
				ai = "";
				while(/\d/.test(fmt[i]))
					ai += fmt[i++];
				ret += ai !== "" ? argv[ai] : fmt[i-1];
				--i;
				continue;
			}
		}
		ret += fmt[i];
	}
	return ret;
}

/* export the module (e.g. in React) */
//export {t9n_use,t9n_format,_}

React Native sample usage:

/* translations */
import {t9n_use,_} from "./src/t9n";
import it_IT from "./languages/it_IT";

global._ = _; /* t9n._ anywhere */

const Languages = [
        {name: "Italiano", locale: "it_IT", data: it_IT},
];

The file ./languages/it_IT.js is like the following:

export default {
        getpidx: (n) => n == 1 ? 0 : 1,
        catalog: {
                SLOGAN: "t9n rocks (it_IT)",
                /* ... */
        }
};

A getlocale implementation may looks like this:

import {NativeModules} from "react-native";

function getlocale() {
        let locid;

        if(Platform.OS == "ios") {
                locid = NativeModules.SettingsManager.settings.AppleLanguages[0];
                locid = locid.replace('-', '_'); /* always use the same format */
        }
        else
                locid = NativeModules.I18nManager.localeIdentifier;
        return locid;
}

Then at some point during startup (possibly before the first render, in App constructor):

/* set the App language */
const locale = getlocale();
const lang = Languages.find(x => x.locale == locale) || Languages[0];
t9n_use(lang.data);

This take the language from the OS locale. If language is not supported then take the first element from Languages. You may also want to check user preferences if any and force the specified language instead.

Now you can use the _ function to translate anything in the app.