Using MusicBrainz-Picard-1.3.0dev20140221.dmg from http://build.oxygene.sk/job/package-picard-osx-daily/17/ again, I was looking at the preferences...

The list on the left is title case, except for "File naming".

The checkboxes use sentence case, except for "Web Proxy", "Browser Integration" and "Tagger Script" (although the latter could perhaps be considered a proper noun).

The "OK" button says "Ok"

The headings for the fieldsets are inconsistent. In title case we have:

• MusicBrainz Server
• Account Information
• Custom Fields
• Folksonomy Tags
• Coverart Providers ("cover art" should be two words)
• Audio Fingerprinting
• AcoustID Settings
• CD Lookup

And in sentence case:

• Preferred release types
• Preferred release countries
• Before tagging
• Tag compatibility
• Name files like this
• Advanced options