What s the word I m looking for?

There are three common words used for a person in training: intern, which normally applies to a professional or knowledge-based work (doctor, accountant, etc.); apprentice, which normally applies to the skilled trades (like carpentry, electrician, blacksmith, etc.) where the work is physical but has an artistry (learned skill) of sorts to it; and trainee, which is a generic term for any kind of basic training.

For me, many technician jobs are somewhat on the learned skill side and would generally fit "apprenticeship" rather than simply training, or "traineeship".

apprenticeship

Apprenticeship?

You are comparing apples and oranges...
"Internship" describes the state of temporary training, it does not describe an occupation. You can be an accountant, or a social worker, or a teacher, or a doctor... all are "interns" if they are serving an intership.
You could also use "trainee", and some might say "recruit", "learner", "pupil", or "apprentice". Therefore, a "technician-in-training" could be any of these things.

I don't things are comparable. If you are employed as a technician, then you in employment. If you are a student in employment, as a technician or anything else, you would still be an intern. If you are apprenticed to a firm then you would be on an apprenticeship, but that is a different thing again.

apprenticeship