There’s no definitive way to gauge a language’s difficulty. It depends on context, different components of a language as a whole, the individual language-learner, and to get more specific, it very well can also depend on a particular language’s family (learning a language from the same family as one’s native language is likely going to be easier than one from a different family), it can depend of alphabet (compare learning a language that uses the Latin alphabet vs. a language that uses Cyrillic), the way it’s written (compare left to right vs. right to left, or top to bottom (like Mongolian script), or heavier detail in symbols); similar writing systems between languages are probably going to be easier to learn than forging out and learning an entirely new writing system—learning new symbols and keeping track of which sounds they represent, and other details about writing and learning a language that I’m not considering.
All of these factors, plus others, are subjective in difficulty—what’s difficult for one may not be for another.
Similarities among vocabularies make related languages easier to learn in that regard. If one can start seeing patterns and similarities, learning the vocab may come a little easier.
-compare English “father” to German “vater” to Dutch “vader.” Even extending from there, outside of Germanic languages but still Indo-European languages, Spanish and Italian “padre,” and Latin “pater.” For some, those similarities are easier to determine than they are for others. But take, for instance, the Russian word for “father”—отец—pronounced “ah-TYEHTS,” and consider arbitrariness of symbols representing sounds, and also consider that Russian uses cases. For many English speakers, this is going to be much more difficult than it would be for some who’s native language is Ukrainian.
Consider, too, that in most languages, there are thousands and thousands of words. Learning a few probably isn’t too difficult for most people, but think about how many different words we use each day in different contexts, and the quantity of words in our everyday vocabulary would take quite a bit of time to learn in another language. And because there are so many words, this generally takes the longest to learn. Even for native English speakers, most of us don’t know quite a few words in our own native lexicon.
Next, we consider rules of usage—politeness, etiquette, informality vs. formality and when to use either, terms of endearment, appropriateness, what is and is not acceptable and the contexts which determine that, etc. I can’t say with any certainty how similar or different these are among all the languages and cultures of the world, but I have to think there are some pretty major differences.
-let’s take one example: formality and informality, as well as plurality, in the second-person in Spanish. Aside from learning new vocab—four distinct words for “you” in Spanish (“tú,” “usted,” “vosotros/vosotras,” and “ustedes”) vs. one in English—one also has to determine the context in which to be formal or informal.
-in this regard, Spanish is more difficult than English because there are more words to learn and more contexts to worry about, but we can also say that English is more difficult because there’s less specificity in both vocabulary and context (which makes the rather stigmatized “y’all” seem like a better idea in this regard).
-Also learning how to pick up on subtexts, reading between the lines, sarcasm, etc.
Let’s consider some aspects of grammar next. For example, the textbook mentions that Hawaiian has only thirteen distinct sounds—eight consonants and five vowels. So learning those sounds is probably easier than learning a language with lots and lots of sounds, but looking at how these phonemes are combined into morphemes may give us a different perspective and throws the concept of “easier” into the air. Because there are so few distinct phonemes, there is going to be quite a bit of phoneme combination into distinct morphemes, which can probably be confusing. For instance,
-“Makai” means “towards the sea”
-“Mauka” means “towards the mountain”
-“Akamai” means “smart/clever”
-“Maikai” “excellent/goodness”
(courtesy of http://hawaiian-words.com/)
-fewer phonemes is probably easier, but keeping them all straight when they’re combined into morphemes is a challenge all on its own.
Analytic vs. Synthetic Languages
-Analytic—word order is how we determine meaning
-Synthetic—affixes, inflection, and cases determine meaning. Word order is not as important in synthetic languages as it is in analytic.
-analytic is generally easier in the process of language learning—as the textbook mentions, children learn language more analytically first, then learn whatever synthetic elements the language has, and when pidgins form, they’re generally more analytic.
-when we start getting into case and gender, things can also get complicated, depending on the emphasis given to them in one’s native language. Consider the definite article, for example. In Modern English, it’s just “the.” That’s it. But in a language like Old English, where case has much more emphasis, there are multiple definite articles that all mean “the,” but depending on whether the word it refers to is masculine, feminine, or neuter, or singular or plural, and depending on case, the definite article is going to be different.
Here are just the masculine singular definite articles in Old English
Nominative—sé
Accusative—Þone
Genitive—Þæs
Dative– Þǣm or Þām
And in German
Nominative—der
Accusative—den
Genitive—des
Dative—dem
For someone whose native language is Modern English, learning about case, gender, and number can be a challenge, and memorizing however many words that all mean the same thing but are different if certain grammatical aspects like case are involved, it’s going to be tough. Or if someone whose native language has no articles, learning a language that has just a few articles, or lots, is going to be tough nonetheless—we can probably assume one would be easier than the other, though.
To conclude: I didn’t cover everything, not even close, but this (I hope, anyway), provides more of a sense that there is no set criteria to determine the overall difficulty of learning another language. It depends on the person, it can depend on the language and its family, but, to offer a glimmer of hope, any language is learnable if one really wants to learn it.
Matters about Language, Poetics, Meaning, etc. 
