Currently you can achieve that by following the tip (Hold CTRL/Command to select multiple labels), but generally some grouping of keywords where within a group clicking would switch the keywords, but between the groups it would "add" the keywords. Or something.
With the new corplist (https://github.com/ufal/lindat-kontext/compare/2018-12-10_on_production...ufal:2019-02-04_on_production) a nice improvement to the keywords would be to have "current version" always selected even if you select a "language keyword" (reported by @vidiecan ).
Currently you can achieve that by following the tip (Hold CTRL/Command to select multiple labels), but generally some grouping of keywords where within a group clicking would switch the keywords, but between the groups it would "add" the keywords. Or something.