The nice thing about French, from an English speaker's perspective, is that its lexical structure is so much like that of English that you can often translate a sentence without having to think much about what it means. Let's try this sentence, for example:
"Process and Reality presents a system of speculative philosophy which is based on a categorical scheme of investigation designed to explain how concrete aspects of human experience can provide a foundation for our understanding of reality."
Without seriously contemplating whatever it is that the author of this sentence is trying to say, I can render this in French as:
"Procès et Réalité présente un système de philosophie spéculative qui est fondé s'appuie sur un plan catégorique d'investigation destiné qui vise à expliquer comment des aspects concrets de l'expérience humaine peuvent fournir une base pour notre compréhension de la réalité."
No doubt there are some issues with this translation – my French has a long way to go. (fixed) But producing it was a relatively easy, almost mechanical task. Translating it into Standard Arabic I have to think a good deal more about the sense of each word (and also have less confidence in the results since I don't own a philosophy-focused dictionary) but I can still readily make it nearly word-for-word:
"كتاب السيْر والواقع يقدم نظام فلسفة نظرية مبني على مشروع فحص تصنيفي معمول ليفسر كيف يمكن لبعض الجوانب الملموسة لتجربة الإنسان أن تعطينا أساسا لفهم الواقع.
("kitābu s-sayri wa-l-wāqiʕ yuqaddimu niđ̣āma falsafatin nađ̣ariyyatin mabniyyun ʕalā mašrūʕi faħṣin taṣnīfiyyin li-yufassira kayfa yumkinu li-baʕđ̣i l-jawānibi l-malmūsati li-tajribati l-'insāni 'an taʕṭiyanā 'asāsan li-fahmi l-wāqiʕi.")
Now suppose I want to translate this into Algerian Arabic. What am I going to do about words like "process", "reality", "speculative", "concrete"? Plenty of Algerians have studied such notions, but they've done so in French or in Standard Arabic. What I would normally do in such cases is simply substitute a Standard Arabic word wherever I can't think of one that would count as Algerian Arabic, yielding something like this:
"كتاب السير والواقع يقدّم واحد النظام تاع الفلسفة النظرية اللي مبنية على مشروع تصنيفي تاع الفحص، خدمُه باش يفسّر كيفاش الجوانب الملموسة نتاع تجربة الإنسان تقدر تعطيلنا أساس باش نفّهمو الواقع."
("ktab əs-sayr w-əl-wāqiʕ yqəddəm waħəd ən-niđ̣am taʕ əl-fəlsafa n-nađ̣aṛiyya lli məbniyya ʕla məšṛuʕ təṣnifi taʕ əl-fəḥṣ, xədmu baš yfəssər kifaš əl-jawanib əl-məlmusa ntaʕ təjribt-əl-'insan təqdər təʕṭi-lna 'asas baš nəffəhmu əl-wāqiʕ.")
On the other hand, what a lot of other educated Algerians would do is something more like this, filling in all the gaps from French:
"كتاب بروسي إي رياليتي يقدّم واحد السيستام تاع لا فيلوزوفي تيوريك اللي مبنية على أن پلان كاتيڤوريك دانفيستيڤاسيون خدمُه باش يفسّر كيفاش ليزاسپي كونكري نتاع ليكسبيريانس إيمان يقدرو يعطولنا إين باز باش نفّهمو لا رياليتي."
("ktab pRose e Reạlite yqəddəm waħəd əs-sistam taʕ lạ-filozofi teoRik əlli məbniyya ʕla ãn plõ kạtegoRik d-ãvestigasyõ xədmu baš yfəssər kifaš lizạspe konkRe ntaʕ l-ekspeRyõs üman yəqqədru yəʕṭu-lna ün bạz baš nəffəhmu lạ-Reạlite.")
Neither of these rather macaronic passages would be comprehensible to any monolingual speaker of Algerian Arabic; they're essentially parasitic on the speaker's knowledge of Standard Arabic or French. Granted, probably most Algerian Arabic speakers are not really monolingual; but even then, there is no guarantee that a speaker who understands one version will understand the other. If you really wanted to produce a consensus-friendly Algerian Arabic version, that a monolingual speaker would understand – then, basically, you need to completely rephrase the whole sentence to explain these notions in advance. And before I can do that, I need a clearer notion of what the writer means by things like "concrete aspects of human experience". My job has morphed into something that's not so much translation as totally rewriting, and frankly, for a sentence like this I'm not even willing to try it.
Now suppose you're dealing with a language none of whose speakers have ever studied academic philosophy, or for that matter gotten into high school. You can no longer expect to get away with the dodge of code-switching at appropriate moments. How much effort do you think it would take to translate this sentence, compared with the amount of effort it takes to translate it into French? What effect do you think this would have in practice on the cross-cultural transmission of such ideas?
That's one reason why having "no word for X" can matter. The absence of the word – or more precisely, of a fixed expression for it – impedes translation, and hence impedes the transmission of foreign ideas to monolingual speakers. And fixing the problem isn't just a matter of inventing or borrowing a word; to be able to do either, you need to have formulated the corresponding concept, and, in the case of abstract words like these, that presupposes putting a lot of speakers into an originally foreign system of education, with a lot of associated time and expense and all-round hassle.
(Chain of thought prompted by How would you say that in Derja?).