Fix blank web page when locale is invalid by improving language fallback logic

[tjementum]

Summary & Motivation

Fix blank page being served where browser language settings like en or en-UK were uncritically used as the HTML Document language, even if translations weren't available for that exact locale. Now, the logic checks if a translation exists for the base language code (e.g., en), and if none is found, it falls back to en-US. This ensures that the document always has a valid locale that corresponds to available translations.

Checklist

• [x] I have added a Label to the pull-request
• [x] I have added tests, and done manual regression tests
• [x] I have updated the documentation, if necessary

[linear[bot]]

PP-216 Blank web page when locale is invalid

[sonarcloud[bot]]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

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

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