i looked into this, it almost all comes down to this: translating note fields in rpg maker games is almost always a bad idea. here's how to solve all the problems reported above (there might be more, again, translating note names breaks all kinds of stuff)
on major issue 1 (element proficiency doesnt get added by items, and doesnt work on attacks), you've renamed the element names (炎->Fire), but it is relying on there being a tag like <炎の知識:+20%> to increase fire damage by 20%, and that 炎 is referencing right into the elements list, which it cant find now that you've renamed the element name.
The same thing happens on attacks, where attacks have 炎 in the notes field to indicate that they are affected by fire proficiency, but now this says "flames" which doesnt match the element name anymore (Fire), so that doesnt work either, making attacks not take into account the proficiency tags set above. (aka no extra damage)
on major issue 2 (cant sell momo items), you translated<Rgroup:ももも商店> as <Rgroup:Momo Momo Shop>, but this addon for restricted shops only works with 1 word group names (you'll need to use MomoMomoShop instead), you'll also have to change the script/trigger on the momo shop map on this line "Rshop Momo Momo Shop" to match the new name (Rshop MomoMomoShop)
on major issue 3 (weapon upgrade attacks dont work), this is because in the jp note, it uses this kind of code <CAG攻撃:326> which sets the attack to "ice storm", but your translated version uses <CAGattack:326>, while the script is looking for <CAGAttack:326> (note the capital A), this causes it to not recognize the attacks
you'll either need to go in to all the note fields of every item and skill and modify these, or put them back to how they were in japanese to make these work
i hope this helps!
Here, I tried to fix them up. I cant guarantee this will work in all cases, but it seems to work as expected and solves all the major problems reported on this page
