WordPress Version 4.5
CMB2 Version 2.2.1
When registering a select list as a repeatable field, changing the add/remove button text adds the button text as an option to the select list because you have to change the text in the options argument which is where the options for the select list live.
Here's a gist showing the issue: https://gist.github.com/brandondove/5e72f7497bbb01a6c1962c9d3846827e
Here's a screenshot of the result:
