"'"
The {{" ' "}} (or {{Double single double}} in long form) template is for use a) at the beginning of a "double-quoted" quotation that begins a 'single-quoted' quotation that in turn begins an enclosing "double-quoted" quotation, or b) at the end of a "double-quoted" quotation that ends a 'single-quoted' quotation that in turn ends an enclosing "double-quoted" quotation. Its purpose is to slightly space the single and double quotation marks apart for legibility, without actually inserting a space character. It does this with CSS, and does so because the insertion of extraneous space characters of any kind would be a violation of the semantic integrity of the web content of the article or other page in which it appeared.
Kwan was quoted as saying, "She disputed his statement that 'Voltaire never said "I disapprove of what you say, but I will defend to the death your right to say it.{{" ' "}}
Kwan was quoted as saying, "She disputed his statement that 'Voltaire never said "I disapprove of what you say, but I will defend to the death your right to say it."'"
Kwan was quoted as saying, "She disputed his statement that 'Voltaire never said "I disapprove of what you say, but I will defend to the death your right to say it." ' "
On some Mozilla-based browsers, the spacing is not perfectly even on both sides of the single-quote. There isn't anything we can do about this without making it look equally imperfect in most or all other browsers. This is a Mozilla bug, not a MediaWiki bug.