(core) Fix missing placeholder flag icon

Summary:
Shows a placeholder flag icon for the language picker button when a country flag
isn't available.

The country flag icon is displayed on top of the placeholder icon. For countries
where an icon isn't available, the placeholder will then become visible.

This fixes a bug where no icon was shown for languages that didn't have a flag
icon available.

Test Plan: Tested manually.

Reviewers: jarek

Reviewed By: jarek

Subscribers: jarek

Differential Revision: https://phab.getgrist.com/D3805

test/nbrowser/LanguageSettings.ts

@@ -34,7 +34,7 @@ describe("LanguageSettings", function() {
         const button = await langButton();
         assert.isTrue(await button.isDisplayed());
         // Make sure correct flag is shown.
-        const flag = await button.find("div").getCssValue("background-image");
+        const flag = await button.find(".test-language-button-icon").getCssValue("background-image");
         assert.isTrue(flag.endsWith(countryCode + '.svg")'), `Flag is ${flag} search for ${countryCode}`);
         // Make sure we see the all languages in the menu.
         await button.click();