I would assume there’s some kind of background check, they don’t want russian spies to join. And if you join the army you don’t necessarily get to choose what your assignment will be.

What roles can foreigners occupy in the Ukrainian military?

Currently, the greatest need is for infantry, specifically riflemen. If you have other specialized skills, you may be considered for different roles. Many positions, particularly in the air force, air defense, logistics, and medical specialties (excluding combat medic), require knowledge of the Ukrainian language, which may limit eligibility. Your specific position will generally be determined based on the abilities demonstrated during the training process. However, if you have preferences, please include them in your email application.

Emphasis added. Source (see “Joining”, #7).

You may want to consider checking with the hospitallers, that’s how brit Brandon Mitchell got his way in