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

@@ -8,3 +8,5 @@ def test_hello(client):
     response = client.get('/hello', headers={'Accept-Language': 'en_US'})
     assert response.data == b'Hello, World!'
 
+def client(app):
+    return app.test_client()
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