What's changed since the last time this was posted: APL is now generally free. J[1] is GPL. The newest version of k is closed-source, but still freely available for personal use from shakti[2], and there is also an independent AGPL implementation[3]. Dyalog APL[4] is closed-source but free for personal use; and, finally, GNU APL[5] has always been open-source, but it's matured quite a bit. Whatever your choice, you have a freely usable option.

1: https://www.jsoftware.com/

2: https://shakti.com/

3: https://bitbucket.org/ngn/k/src/master/

4: https://www.dyalog.com/

5: https://www.gnu.org/software/apl/