Localization support

Localization support. Finnish language added as a second language for the system. Also fixed an issue that could have shown direct error messages to the end users.

Needs testing before merge

tests/test_example.py

 from tjts5901 import create_app
+from tests.conftests import client, app
+
 
 def test_config():
     assert not create_app().testing
@@ -6,5 +8,4 @@ def test_config():
 
 def test_hello(client):
     response = client.get('/hello', headers={'Accept-Language': 'en_US'})
-    assert response.data == b'Hello, World!'
-
+    assert response.data == b'Hello, World!'
\ No newline at end of file