Improve typing of lang.py, and ensure Python 3.8 compatibility

kanken_online/lang.py

@@ -1,10 +1,11 @@
 import json
 import os
 from pathlib import Path
+from typing import Dict, Optional
 import click
 from flask import Flask, session, current_app
 
-LanguageMapping = dict[str, str]
+LanguageMapping = Dict[str, str]
 
 def load_language(lang_code: str) -> LanguageMapping:
     with open(Path("kanken_online/static/lang", f"{lang_code}.json")) as f:
@@ -29,11 +30,12 @@ update_languages()
 # assert not any((((key in lang) and (key not in EXISTING_STRINGS)) for key in lang) for lang in LANGUAGES)  # Ensure no languages have strings not specified by the main index
 
 
-def localize(text_id: str, language: dict[str, str] = None) -> str:
+def localize(text_id: str, language: Optional[Dict[str, str]] = None) -> str:
     if language is None:
         preference = session.get("language", "ja")
         language = LANGUAGES.get(preference)
-    return language[text_id]
+        assert language is not None
+    return language.get(text_id, text_id)
 
 
 @click.command("add-string")