Alphabetical index shows concepts and letters in UI language instead of content language.
Link to relevant issue(s), if any
Part of #1563
Description of the changes in this PR
Fetch letters and concepts based on content languge, addresses requirement 3 in #1563
Known problems or uncertainties in this PR
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)
Reasons for creating this PR
Alphabetical index shows concepts and letters in UI language instead of content language.
Link to relevant issue(s), if any
Description of the changes in this PR
Fetch letters and concepts based on content languge, addresses requirement 3 in #1563
Known problems or uncertainties in this PR
Checklist
.sr-only
class, color contrast)