I originally learned the word through a saying: ‘I thank Thee, Lord, for not being a москаль!’ (Moskál' is a slur aimed at Russians)
I hear the original sense of moskal' was "a soldier of the Russian tsar's army", so maybe at first it meant "thank God I'm not conscripted"?
It's not the original sense. Москаль comes from widely used in XV-XVII centuries name of Russia: Московія, Московська держава. So the state was called Московія and its citizens - москалі. Since Ukraine became the part of Russian Empire people started to use this word for soldiers (as soldiers were the biggest social group of Russians in Ukraine). People used to say something like "забрали у москалі" about the Ukranian recruiters to the Russian army. But eventually it lost its "military" meaning and people again started to use for Russians
'Будь ласка' & 'Прошу' are Ukrainian terms for 'Please'.
You can REPLY to 'Дякую' with: 'Прошу!', 'Прошу дуже.' and/or 'Нема за що!' ('It's nothing!')
My fave is:"Моя приємність!" ("My pleasure!")
I speak Czech and Slovak too, there are Děkuji a Ďakujem, so in all Slavic languages are similar.
And also in the Lithuanian language, you'd say, "dėkuyė!" (I think I spelled that wrong. If so, please tell me the correct spelling on Google Translate or your knowledge of Lithuanian.) ;)