I think characters via alphabets dwarfs everything else for written language, but here are a few other factors:
how phonetic it is if you want to learn the writing system.
how regular it is—English is full of exceptions (because of its 3 language family history). Spanish is very regular, Russian also has a ton of exceptions.
how complicated the morphology is (Finnish is tough because of the super complex morphology, see also Russian). Mandarin is very easy on this dimension.
whether there’s a phoneme distinction that you didn’t learn as a child—so for a Japanese speaker, l vs r is hard in English (“Engrish”), and for an English speaker, o versus ō is hard in Japanese.
In general, of course, the more similar it is to your native tongue, the easier it is. Tones are hard to learn if you don’t speak a tonal language, but if you do then they are super intuitive. Similar with lots of morphology, fixed versus fluid word orderings, etc.
The other angle is spoken versus heard. Portuguese (especially) and French are much easier to speak than understand because of the various ways that sounds are elided or mushed together with fluent speakers. So you can get basic sentences out before you can understand something at full speed—generally true but much more so for some languages.
I think characters via alphabets dwarfs everything else for written language, but here are a few other factors:
how phonetic it is if you want to learn the writing system.
how regular it is—English is full of exceptions (because of its 3 language family history). Spanish is very regular, Russian also has a ton of exceptions.
how complicated the morphology is (Finnish is tough because of the super complex morphology, see also Russian). Mandarin is very easy on this dimension.
whether there’s a phoneme distinction that you didn’t learn as a child—so for a Japanese speaker, l vs r is hard in English (“Engrish”), and for an English speaker, o versus ō is hard in Japanese.
In general, of course, the more similar it is to your native tongue, the easier it is. Tones are hard to learn if you don’t speak a tonal language, but if you do then they are super intuitive. Similar with lots of morphology, fixed versus fluid word orderings, etc.
The other angle is spoken versus heard. Portuguese (especially) and French are much easier to speak than understand because of the various ways that sounds are elided or mushed together with fluent speakers. So you can get basic sentences out before you can understand something at full speed—generally true but much more so for some languages.