relay client's preferred language in link preview requests

Closes #1440.

src/plugins/irc-events/link.js

@@ -38,7 +38,7 @@ module.exports = function(client, chan, msg) {
 	})).slice(0, 5); // Only preview the first 5 URLs in message to avoid abuse
 
 	msg.previews.forEach((preview) => {
-		fetch(preview.link, function(res, err) {
+		fetch(preview.link, {language: client.language}, function(res, err) {
 			if (err) {
 				preview.type = "error";
 				preview.error = "message";
@@ -85,7 +85,7 @@ function parse(msg, preview, res, client) {
 
 		// Verify that thumbnail pic exists and is under allowed size
 		if (preview.thumb.length) {
-			fetch(escapeHeader(preview.thumb), (resThumb) => {
+			fetch(escapeHeader(preview.thumb), {language: client.language}, (resThumb) => {
 				if (resThumb === null
 				|| !(/^image\/.+/.test(resThumb.type))
 				|| resThumb.size > (Helper.config.prefetchMaxImageSize * 1024)) {
@@ -198,7 +198,19 @@ function emitPreview(client, msg, preview) {
 	});
 }
 
-function fetch(uri, cb) {
+function getRequestHeaders(language) {
+	const headers = {
+		"User-Agent": "Mozilla/5.0 (compatible; The Lounge IRC Client; +https://github.com/thelounge/lounge)",
+	};
+
+	if (language !== null) {
+		headers["Accept-Language"] = language;
+	}
+
+	return headers;
+}
+
+function fetch(uri, {language}, cb) {
 	let req;
 
 	try {
@@ -206,9 +218,7 @@ function fetch(uri, cb) {
 			url: uri,
 			maxRedirects: 5,
 			timeout: 5000,
-			headers: {
-				"User-Agent": "Mozilla/5.0 (compatible; The Lounge IRC Client; +https://github.com/thelounge/lounge)",
-			},
+			headers: getRequestHeaders(language),
 		});
 	} catch (e) {
 		return cb(null, e);

src/server.js

@@ -204,6 +204,17 @@ module.exports = function() {
 	return server;
 };
 
+function getClientLanguage(socket) {
+	const acceptLanguage = socket.handshake.headers["accept-language"];
+
+	if (typeof acceptLanguage === "string" && /^[\x00-\x7F]{1,50}$/.test(acceptLanguage)) {
+		// only allow ASCII strings between 1-50 characters in length
+		return acceptLanguage;
+	}
+
+	return null;
+}
+
 function getClientIp(socket) {
 	let ip = socket.handshake.address;
 
@@ -549,6 +560,7 @@ function performAuthentication(data) {
 		socket.emit("configuration", getClientConfiguration());
 
 		client.ip = getClientIp(socket);
+		client.language = getClientLanguage(socket);
 
 		// If webirc is enabled perform reverse dns lookup
 		if (Helper.config.webirc === null) {

test/plugins/link.js

@@ -266,4 +266,21 @@ describe("Link plugin", function() {
 			}
 		});
 	});
+
+	it("should use client's preferred language as Accept-Language header", function(done) {
+		const language = "sv,en-GB;q=0.9,en;q=0.8";
+		this.irc.language = language;
+
+		app.get("/language-check", function(req, res) {
+			expect(req.headers["accept-language"]).to.equal(language);
+			res.send();
+			done();
+		});
+
+		const message = this.irc.createMessage({
+			text: "http://localhost:9002/language-check",
+		});
+
+		link(this.irc, this.network.channels[0], message);
+	});
 });