When adding characters to the WikiEditor toolbar, it's possible to set titleMsg, but it's not possible to set a free-form string title. This is rather limiting for personal or site-local scripts which might wish to use a title tooltip but not be able to set a MediaWiki message.
For example, rather than:
{ label: '·', titleMsg: 'special-characters-title-interpunct', action: { type: 'replace', options: { peri: '·', selectPeri: false } } }
you should be able to do
{ label: '·', title: 'interpunct', action: { type: 'replace', options: { peri: '·', selectPeri: false } } }