From ConTeXt wiki

‘quotesingle’ (Unicode character 0x0027) is the unsexed single quotation mark (or quote, or apostrophe) character '. Putting this character in a ConTeXt (or other TeX) source file will usually result in the the ’ (quoteright) character in the output. Usually, this is what you want: it lets you use the more easily typed ' as an apostrophe in the source file, and have it typeset as the more aesthetically pleasing ’.

However, this causes problems if you actually want the ' character itself in the output. In MkIV, for OpenType fonts, the substitution is controlled by a non-standard OTF feature called trep (short for ‘TeX replacements’). So we can get the ' character throughout the document by ensuring that this feature is switched off in the defaults:

A straight quotation mark: '

But perhaps you want TeX replacement on by default so you can keep getting pretty apostrophes, and you just need the quotesingle character occasionally. In this case you can define a special feature set and activate it only when necessary.

A straight quotation mark: \realquotesingle .
A normal quotation mark: '.

Note that ‘mode=node’ is necessary for this to work. Also, as stated, this technique applies only to OTF fonts under MkIV. See also this mailing list discussion.