[Fixes #14659] Error in uploaded metadata in multilang instances

[etj]

Forward porting of #14668

(cherry picked from commit 041e525)

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[Copilot]

Copilot review overview

🟢 Approval recommended

No unresolved review comments remain, and approval assessments are unanimous.

Review effort: Lite
Findings: None

What changed in this PR

Fixes metadata loss during monolingual partial uploads in multilingual GeoNode instances.

Changes:

• Preserves base-field values as the default-language translation.
• Adds regression tests for multilingual-aware and unaware updates.

File	Description
geonode/​metadata/​tests/​test_multilang.py	Adds regression coverage for partial metadata updates.
geonode/​metadata/​handlers/​multilang.py	Updates multilingual deserialization behavior.

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