Even though I know the correct meanings, no matter how hard I try the word “flammable” sounds to me like “able to flame” while “inflammable” sounds like “incapable of flame”.
English is beautiful. I say that as an ESL who speaks Spanish and Italian fluently.
English spelling, and etymology are fascinating and closely related. English spelling often retains the original spelling despite changes in pronunciation.
Inflammable come directly from the Latin where “in” is not a negation but putting something “in” a state of being, in this case flame.
Beauty is in the eye of the beholder. Thus I have no business telling you whether you are wrong or not in your assertion.
However, both its grammar and spelling is incredibly ad-hoc. To someone who is a fan of structure (such as a programmer like myself), this is frustrating. I was going to make a comparison to a programming language, but I came to the conclusion that there isn't really a programming language out there that absorbs everything without making it natural in the adopted framework.
Like you, I learned English as a second language and even though I can speak it reasonably well, I'm constantly annoyed by its idiosyncrasies.
Inflammable means able to be put “in flames”. But most people read “in-“ in this case as a negation, and therefore assume it’s the opposite.
So “flammable” is born and people start using it and thinking the original form is opposite.
But inflammable materials are dangerous to treat otherwise! Since no one confuses “flammable” for inert, for safety reasons, the word “flammable” is used when labeling inflammable material.
The Simpsons made a joke about it once with Dr. Nik.
Which has the same definition as inflammable.
This has always been inconceivable in my mind.