There is a controversy among language enthusiasts: is it possible for a language to be objectively easier than another?
I would argue Yes. The first objection is usually (and it is correct) that for a speaker of Cantonese, Mandarin is easier than English, while for a speaker of German, English is easier than Mandarin. So obviously the comparison needs to be made under the condition that all else is equal: given two languages that are both equally familiar or equally unfamiliar to the person in question, it is possible to say that one is easier than the other.
Take two Slavic languages for example, Russian and Croatian, and assume a learner who has never studied any Slavic language. I believe that in this case, it is possible to say that Russian is objectively harder because its spelling and pronunciation are less straightforward, while both languages feature a similar a relatively complex grammar and rich vocabulary.
The writing system itself need not be a reason to consider a language to be harder than another, provided the spelling is reasonably phonetic. That is, learning "Korean spelled in Latin letters" would take a few hours less than "Korean spelled in the Hangul alphabet", but as long as the writing system can be learned in a few hours (through my Script Hacking books for example), we need to examine whether the pronunciation, grammar and vocabulary are significantly easier than in the other language, so as to offset the extra few hours spent on the writing system.
And this is where it can get tricky. Often there seems to be some kind of balancing, that is, languages that are unable to express as many nuances through morphology (prefixes and suffixes) instead express these nuances through more rigid word order, more rigidity as to how something "has to be said", and a more extensive vocabulary. How does this look concretely?
Grammar vs. word order
Latin word order was extremely free, at the cost of having to memorise tables upon tables of endings that tell you what a word does in the sentence. By contrast, English has next to no endings, but its word order is very rigid. Poetry written in Latin, or in other languages that have a free word order, loses most of its power when translated into English and many people who have experienced poetry in one of these languages will prefer it over poetry in English.
Grammar vs. vocabulary
Another example: many learners of German dread the system of separable and non-separable verb prefixes. It is a part of grammar that is obviously harder than e.g. French grammar. However, there is a cost in vocabulary: learners of French have to learn a lot more word roots because French does not extensively use verb prefixes. Compare:
German: gehen (go), ausgehen (out-go), mitgehen (with-go), vorgehen (front-go), vorbeigehen (past-go), entgehen (de-go), zergehen (dis-go) --> learn only 1 root: gehen
English: go, go out, go along, go ahead, pass, escape, dissolve --> learn 4 roots: go, pass, escape, solve
French: aller , sortir, accompagner, avancer, passer, échapper, fondre --> learn each of the 7 roots
Doing the math
How do you weigh this against each other? I don't think it's possible. People with a very logical brain may prefer the logic of Latin and German systems and learn these more quickly than the randomness of French, while people whose brain is less attuned to tables and patterns may be overwhelmed by Latin and German and find French easier. So even when considering only English speakers with a comparable background in language-learning, it is not clear which of these languages will require the least amount of hours.
Add to that the difference in learning goals and study materials. Latin is generally learned with an emphasis on passive understanding, while you cannot claim to know French unless you know how to read AND speak it. In Latin, you're expected to be able read literature in order to pass the final exam, meaning that your vocabulary has to be quite extensive, while you may be able to pass French class with a much smaller vocabulary as long as you can use it effectively in conversation. The materials are also not comparable. Languages like Russian and German have a huge amount of different materials (textbooks, easy readers, videos, podcasts, apps...) catering to all levels and all study styles, while the selection of Croatian and Danish learning materials is much more limited, meaning that you may not learn at the optimal pace.
I still believe that it is possible to say that e.g. for a monolingual English speaker, the overall difficulty looks roughly like this, from easiest to hardest:
Esperanto < Dutch < German < Russian < Chinese
That is, to describe difficulty in rough strokes. Beyond that, it really depends on personal factors:
- Most of us have had at least some language classes at school or even learned one or more languages to a conversational level. That will completely warp the picture of which language is easiest to learn next.
- If one of the languages you're considering might be deemed difficult due to comparative lack of materials (say, Javanese), you can mostly ignore that if you live in the area where it's spoken - or if you like to study languages the old-fashioned way and don't depend on Netflix, easy readers etc.
- What do you personally find easier to learn: a language with harder grammar, harder vocabulary, harder spelling, or harder pronunciation? This personal aptitude often makes the difference.
The above describes how many hours you'll need to spend on the target language. But you and I know that sometimes an hour feels interminable and sometimes we barely notice it passing. This mainly has to do with motivation. So in the final analysis, whether a language FEELS easy to learn or feels hard also depends on how good your reasons are for learning it. If you need the language to talk to the person you've fallen in love with and you don't have another language in common, that's probably the easiest language you've ever learned.