I'll be the first to agree, learning thai especially as an adult comin' from an english speakin' country is a tough row to hoe. I mean you'll invest hours and HOURS with very little "bang-4-the-baht" in terms of return on investment. It takes a LONG time to get even semi proficient and most people fall off the learning thai train before that happens.
I don't know that I've EVER met another foreigner who spoke thai yet I considered "fluent". I believe fluent is an imaginary place in your mind or in the minds of people who can't speak thai for shit.. It's a mythical term which is bandied about with little idea what it really means..
I mean can you go to the hospital and talk to the doctor in thai about an upcoming brain surgery operation? Can you go to thai court and interact with the attorney's about a case before the judge? Both those things require very different vocabulary and ways of speaking, yet being able to do either still doesn't mean you're fluent. I think you should strive to be "fluid" when you speak. Don't pause at the wrong time, don't hem-n-haw looking for words, etc. That's what makes a foreigner a proficient thai speaker.
You as a non-native speaker of thai are NEVER gonna fool a thai for even a second into thinkin' you're a born-bred-rice fed thai speaker. No matter how much these over complimentary people praise your thai! You're always gonna have some "tell" where they know you're a foreigner speaking thai. It just is what it is. Don't waste a second of your life on it.
As far as the phrase ฝรั่งรู้มาก. It is spoken because the tiger, errr I mean the thai-gurl you're speaking to knows you aren't a ไก่อ่อน gài ɔ̀ɔn soft chicken or "spring chicken" (newbie, easy mark) and instead you know a thing or two about what's what here.
I hardly hear it but I always snap back with คิดว่ารู้มากดีกว่าไม่รู้อะไรเลย ไม่ใช่รึ "I think that knowing a lot's better than not knowing anything, isn't that right?" That's a really snarky come back and will stop almost any thai cold in their tracks!
There is a very narrow demographic of thai woman which doesn't want their foreign b/f or husband to speak/understand thai. If you ever run into it, it should raise HUGE red flagz with you immediately!
Most rank-n-file thaiz are happy as pigs in shit that you can interact with them in thai and don't compel them to speak what passes for engrish.. As I said, I've yet to run into a thai when I was tryin' to get something accomplished who wasn't happy I could conduct business with them in thai. Even my "version of thai", which sounds surprisingly like a hillbilly from Ohio speaking thai, (possibly because I am a hillbilly from Ohio) who just happens to understand/speak thai too! I had to learn thai to live here because I don't have a "long-haired talking thai dictionary" following me around 24/7 translating what I need to say to thaiz in thai. It was learn to speak thai or die.
Oh, here's one, if a thai gurl says that ฝรั่งรู้มาก to you, try เป็นไก่แก่หนังเหนียว ไม่ใช่ไก่อ่อน "I'm an old chicken with tough skin, not a soft chicken." Both ไก่อ่อน gài ɔ̀ɔn (greenhorn) and ไก่แก่หนังเหนียว gài gɛ̀ɛ nǎng nǐao (seasoned vet, experienced) are comparatives every thai in this country knows. (sorry I don't have the karaoke for that one, but it's a good sentence). .