If I want to enter a monastery in Thailand and become a monk, is it a problem if I can't sit in the lotus position for meditation but in the position I posted above? I'm asking this, because I have never seen a monk meditating in a position different from the lotus position.
I suffer from back problems and need to workout regularly in order to reduce the pain in my back. My back muscles need regular workout because they are weak. Is it possible to perform basic exercises in my own room or is it against the vinaya?
While being in a monastery in Thailand, is it possible to improve my skills in english in order to understand the instructions perfectly? I understand english pretty well but I have difficulties in speaking sometimes. I have learned english at school for 3 years only (I've had some help when writing these questions). So my skills are pretty basic. Or is it possible to learn thai?
Thanks in advance for taking your time to answer my questions.
Answering from the Ajahn Tong (Mahasi-like) vipassana lineage I am somewhat familiar with:
Sitting posture is not a problem. Most people in Thailad, as far as I have seen, practice in the "burmese" posture, having legs flat on the floor (one in front of the other one), but some were sitting in the chair (without leaning) for health reasons.
You will be likely a lay practitioner (on 8 precepts) for some time, before you ordain, but the practice is the same anyway. The most important is that you practice and learn to deal with the situation you have, including back pain, skillfully. That might include some exercise, but also facing the pain in meditation. In fact many people think they have back pain because they have weak muscles (exactly as you say), but it is in fact meditation pain, which is very useful (which does not mean you like it) training for you; it goes away after some time. Your teacher will guide you through that territory while on retreat.
Out of the thousands temples in Thailand, only a minority does meditation, and only a few of them has English-speaking sections (I can recommend, from this lineage, the Chom Tong temple, http://www.northernvipassana.org/about.html; the other ones I know of are Wat Rampoeng and Wat Doi Suthep). You don't need extreme language skills to do Vipassana, just enough to understand the instruction; if you learn Thai in long-term, good, but I would not see it as something to start with. In some centers, there might be yogis speaking your language (French?) which could translate for you.
Before being too far in your plans about becoming a monk, you could do retreats (starting with the 2-3 week "basic course") without long travels to Asia for the start, and then see if the practice is suitable for you; I posted some of the centers from this tradition in Europe in this answer; in France, there is a group organizing retreats regularly which may also help you find a suitable place.