There are so many topics here.
the first thing I think of in complexity is the complexity of the grammar. Latin, what little I learned in the one year it was still taught in my high school, has things like six cases for nouns(?). The short pithy Latin sayings are related to the fact that what we use modifiers for in English, they use word forms. (“Romanus eunt domum”, or “People called Romans, they go house”). Similarly, Spanish or French like Latin tend to use endings rather than auxiliary words to signify tenses; plus more forms of the verb for each person and plurals - so grammar is more obvious to those speakers. I know our Spanish teacher complained he had to stop and give a 4-month intensive grammar course to one of his classes, because students from public schools in Ontario at the time did not do a lot of grammar, did not know first-second-third person, or different past tenses, etc. So certain languages, perhaps as a peripheral Sapir-Whorf effect, are more able to teach grammar in passing. However, it makes editing a sentence so much simpler in English that I can change tense, person, etc. with a very minimal edit - so maybe some ambiguity goes a long ways to making a language computer - friendly.
Then there’s cultural concepts. Bushman may not have the word for “train” so translating Potter may be difficult. That extends to further cultural concepts - some cultures may not have a concept of an afterlife, retributive justice, or slavery. For others - for example, if you grow up in a primarily agricultural society, he-ox vs. she-ox is a pretty important concept; in a culture where men and women are separated frequently, gender is very important. Whereas, technical terms can be more or less specific, depending - “evaporate” means something specific in English which has centuries of scientific study, where it might simply translate as “dry up” in a culture where the only thing they know that evaporates is water.
then there are other cultural issues - for example, one review I recall years ago of Disney said that Europeans we much less likely to anthropomorphize animals, so Mickey Mouse and Donald Duck etc. were somewhat alien to the culture; plus a lot of other behaviour seems very odd - children who are disrespectful of parents, for example. Thus the standard Disney movie formula - kids are smarter than adults, animals smarter than people - fails in many other cultures.
There’s the other issue I recall reading - one of Isaac Asmiov’s essays - that English is essentially the Norman French language overlaid on the Germanic Anglo-Saxon original language. So, there tend to be multiple words for everything, and the more latin form is the more formal - breath vs. respire, sweat vs. perspire, …etc.
