As someone who learnt a foreign to fluency as an adult, I am skeptical of the stronger interpretations of the Sapir Whorf hypothesis. I believe a second language is primarily a source of cultural richness (mutual understanding, interesting vocabulary, intriguing turns of phrase, and so forth) - but not a prerequisite for accessing any novel forms of thinking or reasoning, i.e. I can not recall ever feeling like my thought processes or understanding of the world were fundamentally different to my monolingual colleagues.
I think it depends on how close the two languages are, but also I think that the effect is much more powerful when you are a child and your brain is more malleable.
For example, compared to Western languages, Chinese has very little grammar and words that are more ambiguous and with several meanings. This translates to a way of thinking that is more fuzzy, holistic, and high level, relying more on connections (intuitively, when you don't have grammatical constructs telling you the connection between things, you are forced to seek/guess it by yourself)
> I can not recall ever feeling like my thought processes or understanding of the world were fundamentally different to my monolingual colleagues.
As a native German speaker I, for example, for consider thinking in German (regarding the case system that is deeply ingrained into this language) very much like thinking about types in a type system in a programming language; you have to pass an object [pun intended] of the correct type (genus) to a function/predicate so that the code compiles/the sentence is grammatically correct.
This kind of thinking does not work in English because of its different grammatical structure.
Another example: it is very common in German to analyze words part by part to find "sometimes hidden meanings"). Just one trivial example: "entschweben" [to waft away]: if you analyze the construction of this verb, you see that this is the verb "schweben" [to hover] that is modified with a prefix "ent-" that changes its meaning. This prefix thus works like some higher-order function that modifies the functionality of some existing function (in programming) - or in the given case of language, the meaning of a verb. So, one can really say that how I, as a native German speaker, think about my native language daily often corresponds to quite non-trivial programming concepts.
When I attempted a similar analysis of English words to English native speakers, they soon cut me off and explained to me that "this kind of analysis is not how English works or how English native speakers think about their language".
