See for example https://forum.omz-software.com/topic/4695/downloading-nltk-on-an-ipad-complete-newbie-needs-help
You can install nltk. You cannot install scikit. Basically, you cannot install anything based on C/Fortran, etc. That is a limitation imposed by apples kernel signing requirements, there are no workarounds. As @ccc points out, the other actively developed app is Pyto, which you should check out as it includes more built in modules -- however the UI side of things needed some work.