The true test of this theory would be to translate some English poetry into Russian and see if it comes out sounding deeper, or to translate some English postmodernism into French and see if it comes out sounding more authentic.

It's great that you proposed an experiment. Still, the language might facilitate original expression in a certain form more than it facilitates translation. To control for that effect somewhat you could ask a good Russian poet or French philosopher to do the translation, and I can guess how that will turn out!

Can't say about philosophy, but Russians with good knowledge of English or other languages often prefer good Russian translations of literary works to the originals, despite preferring the originals of technical texts etc. Maybe an artefact of the Soviet era when non-state-sanctioned writers often worked as translators, raising the average quality of translations at the cost of never publishing their own thing. For example, I didn't much enjoy the original text of LOTR compared to the Russian translation by Muravjev/Kistyakovsky. The contrast is especially felt in the poetry: Russian has a much deeper store of rhymes, no one is forced to stick to "lie - die" or "land - hand" as Tolkien had to, and all inconsistencies of meter are also gone - I was actually amazed how many of them there are in the English version, making some verses almost unreadable.

Nominull, your remarks about language here and above seem off (although my experience is narrower than cousin_it's, since I'm only bilingual, German is coming slowly, with little motivation). Each language has its sound, influencing the way you can use it for different tasks. Of course, you can accurately communicate a deeply understood concept in any language, by describing it redundantly, but that doesn't apply to the sum total of everyday use, in particular to viewing the language as a tool for refining your concepts.