Translation:Nobody travels up that high mountain.
Does "sem" still translate explicitly as "not ... either" in a sentence like this, or does it roll up into the meaning of "senki"?
The latter. For terms like "nobody", "nowhere", "never", etc., Hungarian uses double negation. "Senki" means nobody, but it must be used with "sem" or "nem" in a sentence. ("senki nem" means the same as "senki sem".)
"high" should be accepted. https://dictionary.cambridge.org/grammar/british-grammar/high-or-tall: We use high for mountains and for things which are a long way above the ground. ... We use tall to describe things which are high and thin in their shape (e.g. buildings, trees).
I put "nobody is travelling up on that high mountain either", but there is no mention of "either" in the Duo translation: am I missing something?
arra = MIA? in that direction?
a = that?
Nobody travels up in that/the direction of the high mountain?
fel arra -re is a little much information/direction?
In this sentence, arra has nothing to to with direction. It is az+ra
az a hegy that mountain
arra a hegyre onto that mountain
it is similar to: azon a hegyen, azzal a heggyel, etc.