The English language usually has slanted integrals that look like this:
Some languages, like German, use upright integrals instead:
When using the default font ‘New Computer Modern Math’, this upright version can be enabled using stylistic set 2 in the text function (ss02 in OpenType parlance).
show math.equation: set text(stylistic-set: 2)
This information was not readily available; I had to dig into the font files and changelogs on ctan to find this feature. So, for the sake of convenience, I believe that setting the language to an upright integral locale, like German, should automatically enable ss02 when using the default font. Or, alternatively, the guide should mention this feature in the maths section and that the upright stylistic set may be present in other fonts too.
