Set the UI language cookie

[joelit]

Reasons for creating this PR

Link to relevant issue(s), if any

• Closes Skosmos 3: Remember UI language using a SKOSMOS_LANGUAGE cookie #1696

Description of the changes in this PR

• Fixed the navbar language menu
• Added functionality for setting the language cookie
• Added tests for setting and reading the language cookie

Known problems or uncertainties in this PR

• The functionality in index.php and web controller seemed to be OK, they might need a tweak in this

Checklist

• phpUnit tests pass locally with my changes
• I have added tests that show that the new code works, or tests are not relevant for this PR (e.g. only HTML/CSS changes)
• The PR doesn't reduce accessibility of the front-end code (e.g. tab focus, scaling to different resolutions, use of .sr-only class, color contrast)
• The PR doesn't introduce unintended code changes (e.g. empty lines or useless reindentation)

[osma]

I added some notes. At least the TTL of the cookie needs to be changed.

[osma]

Looks good now. I made a couple of minor additional suggestions for you to consider.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud