frontend: Add missing translation strings

[shiina424]

Description

This PR adds missing translations that were not included in the following commits:
Existing Button: 5fc2476
Audio Mixer Hidden Button: a42a6c4

If this change is not appropriate, please feel free to close this PR.

Motivation and Context

I want to add the missing translations so these elements can be properly localized.

How Has This Been Tested?

Tested on Windows 11 25H2 using a local build.
I'm not very familiar with the codebase, so I apologize if this change is incorrect.

Types of changes

• Tweak (non-breaking change to improve existing functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.