Thai and English are too extreme languages. Both are difficult.
Thai is much more regular in symbolic representation --we can write what we can say easy - no matter what our emotion may be at the time--.
English is more regular in grammar (rules) and coverage of "essential information"(time, number, gender, and 'mood') but due to extensive "borrowing" of words from many parts or the world over time, not regular to read or write.
Thai gives fun and sensations of being in close encounter or intimate.
English gives us more (peripheral) information including time, number, ... whether we like to know or not. In this way, learning English is learning to think or form a habit to capture more little details or context of information.
We all learn of the Who-Does-What-When-Where-Why-and-how, English re-enforces this style of enquiry. Thai expresses less concerns for time, number, ... but not in the style above.
Here we talk above "habit"-forming languages. How difficult is a language (to learn) may be related to how difficult to change/adapt to new requirements/environment and (maybe) how to make a living.
We can talk more about culture: [knowledge, language, life/livelihood] ;-)