Translate all out-of-game UI (start screen, lobbies, etc.) (#316)

This PR adds full translation support for all out-of-game UI elements,
including the start screen, public/private lobby modals, and other
pre-game interfaces.

All static text has been externalized to en.json and ja.json for future
language support.
If you find any spots that are not yet translated (missing from the
JSON), please let me know.
Thanks a lot!

This is a follow-up to PR
[#305](https://github.com/openfrontio/OpenFrontIO/pull/305).

---------

Co-authored-by: Cldprv <dubois.cnm@tutanota.com>
Co-authored-by: jacks0n <rosty.west89@gmail.com>

src/core/validations/username.ts

@@ -4,6 +4,7 @@ import {
   englishRecommendedTransformers,
 } from "obscenity";
 import { simpleHash } from "../Util";
+import { translateText } from "../../client/Utils";
 
 const matcher = new RegExpMatcher({
   ...englishDataset.build(),
@@ -41,28 +42,33 @@ export function validateUsername(username: string): {
   error?: string;
 } {
   if (typeof username !== "string") {
-    return { isValid: false, error: "Username must be a string." };
+    return { isValid: false, error: translateText("username.not_string") };
   }
 
   if (username.length < MIN_USERNAME_LENGTH) {
     return {
       isValid: false,
-      error: `Username must be at least ${MIN_USERNAME_LENGTH} characters long.`,
+      error: translateText("username.too_short", {
+        min: MIN_USERNAME_LENGTH,
+      }),
     };
   }
 
   if (username.length > MAX_USERNAME_LENGTH) {
     return {
       isValid: false,
-      error: `Username must not exceed ${MAX_USERNAME_LENGTH} characters.`,
+      error: translateText("username.too_long", {
+        max: MAX_USERNAME_LENGTH,
+      }),
     };
   }
 
   if (!validPattern.test(username)) {
     return {
       isValid: false,
-      error:
-        "Username can only contain letters, numbers, spaces, underscores, and [square brackets].",
+      error: translateText("username.invalid_chars", {
+        max: MAX_USERNAME_LENGTH,
+      }),
     };
   }