It seems that the number of words in a language would be a good indicator. However, the average person only knows 20,000 words or less, so it doesn't matter if there are 60,000 or 100,000 or even 200,000 words in a language - they will never use them. Poets have bigger vocabularies, but they cannot use words that are too obscure for their readers, so they never reach this upper limit of language either.
You can train people to be more expressive. ...her small expressive face.
What's interesting are agglutinating languages, where adding prefixes and suffixes can add nuances to any word. Just by recognizing the word root and recognizing the prefixes and suffixes, the average person can thus understand a lot more nuanced ideas, which would require advanced vocabulary or circumlocutions in other languages.
Here's an example using Esperanto:
bela = beautiful
malbela = ugly (mal- for opposites, e. g. mal-bona = bad, mal-helpi = to hinder, mal-lumo = darkness)
belulo = beautiful person (-ul- for people, e. g. help-ulo = helper, bon-ulo = good person, forgesulo = forgetful person)
malbelulo = ugly person (you can combine any amount of prefixes and suffixes)
belulejo = place for beautiful people (-ej- for places, e. g. manĝejo = dining-room, mal-san-ul-ejo = place for sick people = hospital)
malbelulejo = place for ugly people
fibelulo = morally bad but beautiful person (fi- is for morally bad people, e. g. fiŝtelisto = thief, fipolitikisto = corrupt politician, fiulo = villain)
If "belulo" (beautiful person) can still be expressed in English using a single word like "hunk", "belulejo" (place for beautiful people) is a completely new idea ("Italian beaches have the reputation of being sunny belulejoj"), yet this word is very clear to anyone who knows basic Esperanto - I have had beginners come up with words like this after less than two hours of instruction. "fibelulo" (morally bad but beautiful person) is such a useful word it should really exist in other languages, but I don't know any that has it, despite countless world-class novels and plays using this kind of character. "forgesulo" (forgetful person) is also very useful.
And there are other awesome words writers can create on demand, for example "forgesejo" (place of forgetting, e. g. a writer described a boy's pants pockets as a forgesejo, because things accumulate there over time and are forgotten) or "detaladi" (to go over details and more details for a really long time). There are many more prefixes and suffixes as well, for different nuances.
For me, Esperanto is the language that allows me to express nuances best. I don't know any other language with agglutinating features; I imagine it's similar in those. If not agglutinating languages, I'd probably vote for Chinese - I personally cannot express myself that well in it, but I already noticed their habit of coining most of their words based on two different meanings, which can be the same, slightly different or completely different ideas, so that there are dozens of possible nuances where English only has one or two. (c) Judith Meyer who speaks 8+ languages.
у нас к слову -идти- можно добавить где-то 6 приставок,
у них слово -gehen- может принимать 31 приставку, давая новые смыслы...
прекрасный язык для тех, кто копает глубже. * Немецкого языка звучание vs. FR, EN, IT, ES
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* приставка MIS- толкования miss, misuse, misunderstand, misplace, mistake
* Происхождение языка: обзор 6 теорий и мое мнение