Release v1.2.1: mkvmerge v98 language-IETF compatibility

[Hydro74000]

This release merges devel into main for v1.2.1.

Main change confirmed from git history and diff review:

• the dominant update is compatibility with mkvmerge v98
• mkvmerge v98 introduced a breaking change around language-ietf / IETF language tag handling
• this release adapts remux/build/config behavior to that change and fixes language re-emission

Included changes from git history:

• fix: get ICU DLLs from windows, missing in wine/crossbuild
• build update due to mkvmerge v98 changing language-ietf tag mecanisms
• fix titre vide
• fix language reemit due to mkvmerge v98 breaking change

Validation notes:

• targeted tests run locally with python3 -m pytest -q tests/test_remux.py tests/test_encode_workflow.py tests/test_setup_and_config.py tests/test_package.py
• result: 408 passed, 2 failed
• the 2 failures are Windows-specific tests patching ctypes.windll, which is unavailable in this Linux runtime