Check translation contracts offline, add Swedish to the installer and enable the website languages

- Offline checks for terminal and website catalogs reject translations that change commands, paths, identifiers or placeholders
- Terminal and documentation texts that broke those contracts are corrected in every language
- Swedish can be chosen in the stable and beta installers
- The website routes, language switcher and exported pages cover the eight languages, with the shared website texts in each of them
- Website documentation translations are generated only when requested
- From PR #475 by @jcastro

.github/scripts/tests/test_installer_languages.py

@@ -0,0 +1,30 @@
+"""Every shipped terminal language must be selectable during installation."""
+from pathlib import Path
+import re
+import sys
+import unittest
+
+SCRIPTS = Path(__file__).resolve().parents[1]
+ROOT = SCRIPTS.parents[1]
+sys.path.insert(0, str(SCRIPTS))
+from build_translation_cache import DEFAULT_LANGUAGES
+
+
+class InstallerLanguageTests(unittest.TestCase):
+    def test_both_installers_offer_every_shipped_language(self):
+        expected = {'en', *DEFAULT_LANGUAGES}
+        for name in ('install_proxmenux.sh', 'install_proxmenux_beta.sh'):
+            with self.subTest(installer=name):
+                source = (ROOT / name).read_text(encoding='utf-8')
+                function = re.search(r'^select_language\(\) \{(.*?)^\}', source, re.M | re.S)
+                self.assertIsNotNone(function, 'Missing installer language selector')
+                offered = re.findall(r'^\s*"([a-z]{2})"\s+"[^"]+"', function.group(1), re.M)
+                self.assertEqual(set(offered), expected)
+                self.assertEqual(len(offered), len(expected), 'Duplicate language option')
+                # English is the source text in the scripts, not a JSON catalog.
+                for language in set(offered) - {'en'}:
+                    self.assertTrue((ROOT / 'lang' / f'{language}.json').is_file(), language)
+
+
+if __name__ == '__main__':
+    unittest.main()