l10n: add Swedish Cockpit locale

[yeager]

Summary

• add Swedish translation support for the Borg Cockpit interface
• localize the Cockpit language selector and translated UI messages
• add targeted translator coverage

Validation

• python -m py_compile passes for the changed Cockpit modules and test
• .venv/bin/python -m pytest -q src/borg/testsuite/cockpit_translator_test.py src/borg/testsuite/cockpit_test.py — 3 passed, 1 skipped
• The contributor has completed the required human review of this AI-assisted contribution.

[ThomasWaldmann]

Thanks for the PR!

Not sure if the cockpit feature is already developed enough to start a translation effort - or whether translations are wanted at all.

The borg code, docs, help are purely in English yet and offering translations could be a quite wide scoped project with a lot of maintenance burden. Also, the code is changing quite fast for borg2 and esp. the cockpit feature might need a major rewrite (#9454) still that might change a log of strings and how it works overall.

The already existing "translation" feature in the cockpit code was only intended to support switching between the made-up / funny "borg / star trek" language and just boring plain everyday English.

[ThomasWaldmann]

Idea: maybe there should be a comment at a good place in that code stating that to avoid people work on adding translations now.

[PhrozenByte]

If translations are added, I'd recommend using the standard gettext library.

That said, I'm not convinced that translations are necessary either; if the cockpit grows large enough to warrant translations, it might be worth considering moving it into a separate project that uses Borg's JSON API (#9454).

[ThomasWaldmann]

Not planned for now.