"Frauen sollen selbst entscheiden."
Just to build on what freecity wrote... Should is a suggestion, even if it is "made" to yourself. "I should study German more." = "It would be wise if I studied German more."
Shall is more of an order, again, even if it is "made" to yourself. "I shall name my first child Bob, even if it is a girl." A good way to think of the comparison is use the ten commandments. "Thou shall not kill", "thou shall not steal"....I don't know any others... Those are set in stone, this is the law, don't even think about killing or whatever. If the ten commandments said "You know...you really shouldn't steal or kill..." it wouldn't be taken quite as seriously.
In the Duo English sentence, both "shall" and "should" can be used (I don't know if Duo accepts both though). The more common thing you would here would be "Women should decide themselves". For example, not to get political or anything, when talking about abortion, someone is likely to say "women should decide for themselves as to whether or not they should have an abortion". If it was passed as a law that the final decision lay in the hands of the pregnant women, the law would state "women shall decide for themselves...." (the actual write up would be much more legal, but you get the idea).
As for the German use of sollen, I'm still struggling with that a bit myself. I still haven't found a decent explanation as to why "sollten" is "should - in the past" but doesn't write out as "should have".
Just found the following in a german forum. I don't know how accurate it is....
[You are right, "Ich soll mehr Deutsch sprechen" is best translated as "I'm supposed to be speaking more German", which implies someone else is encouraging you.
If you mean to say "I should speak more German", implying that you feel it's the right thing to do, you would say "Ich sollte mehr Deutsch sprechen". It implies you want to speak more German out of your own motivation.
Note, however, that using the Konjunktiv also implies that you're not actually going to speak more German - it only says you feel like you should.
If you want to say that you feel like doing something and you're actually going to do it, you could say "Ich möchte/will/werde mehr Deutsch sprechen" ]