I have been on various long flights (EU<>Asia, EU<>US) and would suggestrecommend you wearingto wear whatever fits you best. Sweatpants are incredibly comfortable, so go for it.
What I recommend as well:
- Wear good shoes, notno flipflops or sandals. In the unlikely event of an emergency this might save your life. Take shoes without laces.
- The same holds true for shirts and pants: wear linen or cotton wool, no synthetic fibers like Nylon that tend to melt on your skin in firecase of extreme heat. Linen and cotton wool also absorb sweat, so the smell after the flight is likely to beprobably better.
- if possible take a fresh T-shirt with you on the plane. Around 1one hour (later the toilets might be crowded) before landing, change your shirt.