Stress is a common problem at work, and using the word correctly is a common problem in English. We use the word ‘stress’ to talk about the thing that causes mental pain, but we do not say we are ‘in stress’, as in Czech.
Instead you have to use the adjectives ‘stressed’ and ‘stressful’. If an individual is feeling under pressure at work, you can say, ‘He is really stressed’. Or if a situation is difficult and making you feel anxious, you can say, ‘It is really stressful.’
A: I am really stressed at the moment.
B: I can understand. That new project you’re doing is really stressful.