I think it depends in your level of intimacy with your teacher, if you two are not that close, then it's better to keep it formal, but if you really want to speak like that, it'd be better if you fixed the sentences: It was pretty **** good! Trying to change my password on this app was a real pain in the neck.

I think it would be beneficial to your English learning to use accurate adjectives along with some acceptable idioms. It was extremely annoying! Trying to change my password on this app was a lesson in frustration! It was exasperating! Trying to change my password on this app was like tearing my hair out!

No - but not because they're not polite. Politeness is not the issue here. 'Pretty damn delicious' just sounds silly, and 'real faff' is a quintessentially British English expression. Both would sound wrong coming from a Russian who (according to his profile) only has a B1 level of English. You need to have near-native proficiency to get away with this kind of language.