nls: add missing localization when preparing the download

[vince-fugnitto]

Bug Description:

The download command is missing a localization when displaying progress regarding preparing the download:

theia/packages/filesystem/src/browser/download/file-download-service.ts

Lines 49 to 59 in ac7a9ff

	async download(uris: URI[], options?: FileDownloadService.DownloadOptions): Promise<void> {
	let cancel = false;
	if (uris.length === 0) {
	return;
	}
	const copyLink = options && options.copyLink ? true : false;
	try {
	const [progress, result] = await Promise.all([
	this.messageService.showProgress({
	text: `Preparing download${copyLink ? ' link' : ''}...`, options: { cancelable: true }
	}, () => { cancel = true; }),

We should properly localize the text as follows:

try {
+   const text = copyLink ?
+       nls.localize('theia/filesystem/prepareDownloadLink', 'Preparing download link...') :
+       nls.localize('theia/filesystem/prepareDownload', 'Preparing download...');
    const [progress, result] = await Promise.all([
        this.messageService.showProgress({
-           text: `Preparing download${copyLink ? ' link' : ''}...`, options: { cancelable: true }
+           text, options: { cancelable: true }
        }, () => { cancel = true; }),

In addition, the default localization for the ProgressMessage.Cancel is missing:

theia/packages/core/src/common/message-service-protocol.ts

Lines 54 to 55 in ac7a9ff

	export namespace ProgressMessage {
	export const Cancel = 'Cancel';

- export const Cancel = 'Cancel';
+ export const Cancel = nls.localizeByDefault('Cancel');

Steps to Reproduce:

1. start the browser application (the download command is not available on electron) with theia as a workpace
2. execute F1 > Configure Display Language, install a new language-pack, and restart the application
3. open the explorer
4. attempt to download a big resource (ex: node_modules) by right-clicking the directory and choosing the "download" command (will be localized)
5. confirm that the progress notification is not localized

Video

download-progress.mov

[vince-fugnitto]

Closed by #11988.