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_i18n.py

+"""
+Translation tests
+=================
+"""
+
+import os
+from typing import List
+
+import pytest
+from flask import Flask
+from flask_babel import (
+    Babel,
+    Locale,
+    force_locale,
+    gettext,
+    get_translations,
+)
+from babel.messages.extract import extract_from_dir
+
+from tjts5901.i18n import SupportedLocales
+from tjts5901 import __file__ as pkg_file
+from conftests import client, app
+
+@pytest.fixture
+def babel(app: Flask):
+    """
+    Babel translation fixture.
+
+    Returns babel tranlaslation fixture registered in flask app
+    """
+    with app.app_context():
+        yield app.extensions['babel'].instance
+
+
+def test_babel_translations(app: Flask, babel: Babel):
+    """
+    Test that translations exists for test string "Hello, world!". This test
+    will fail if the translation is missing for any language.
+
+    This test is not intended to test the translation itself, but rather to
+    ensure that the translation exists.
+
+    And if the actual translation for "Hello, world!" is "Hello, world!", then
+    the test needs to be updated to use a different test string.
+    """
+
+    # For flask_babel to work, we need to run in app context
+    with app.app_context():
+
+        # Iterate through all of the languages available.
+        languages: List[Locale] = babel.list_translations()
+        for locale in languages:
+            if locale.language == "en":
+                # By default everything should be in english
+                continue
+
+            with force_locale(locale):
+                assert gettext("Hello, World!") != "Hello, World!", f"Message is not translated for language {locale.language}"
+
+
+def test_app_language_detection(client, babel):
+    """
+    Similar to :func:`test_babel_translations`, but uses e2e test client
+    to test translations.
+
+    Uses the Accept-Language header to set the language for the request.
+
+    TODO: Write variation that includes the territory code in the Accept-Language header.
+    """
+
+    # Iterate through all of the languages available.
+    with client.application.app_context():
+        languages: List[Locale] = babel.list_translations()
+
+    for locale in languages:
+        if locale.language == "en":
+            # By default everything should be in english
+            continue
+
+        response = client.get('/hello', headers={'Accept-Language': locale.language})
+        resp_as_string = response.data.decode('utf-8')
+        assert gettext("Hello, World!") == resp_as_string, f"Message is not translated for language {locale.language}"
+
+
+@pytest.fixture(scope="session")
+def app_strings():
+    """
+    Fixture for extracting strings from the application source code.
+    """
+
+    # TODO: Read method_map from config file
+    method_map = [
+        ('**.py', 'python'),
+        ('**/templates/**.html', 'jinja2'),
+    ]
+
+    # Collect all of the messages from the source code
+    dir_path = os.path.dirname(pkg_file)
+    messages = set()
+    for msg in extract_from_dir(dir_path, method_map):
+        messages.add(msg[2])
+
+    return messages