One of the css files has the charset definition, which is crucial for preventing the wrong encoding of the symbols and showing weird characters in the slider buttons. But the charset definition in the file is wrong, which causes sometimes a wrong encoding.
In the file inst/htmlwidgets/lib/slick-1.8.1/slick/slick-theme.css, it has @charset 'UTF-8'; in the first line, but as shown here, it needs double quotes.