"Двадцять одна гривня"
Yes, I don't see how cutting and pasting is faster than switching keyboards. You can assign a CTRL + # to each keyboard. Pressing two keys simultaneously is much faster. Of course, I am taking enough courses that I actually hold CTRL+SHIFT+9 for Ukrainian keyboard, but I have English on CTRL+1.
What might be slightly confusing to Polish speakers is in fact not assuming that "одна" means "first" by itself, but the use of singular after numbers that end with 1 (in Polish we use for those the same form we use for numbers ending with 5-9 and 0). This might resemble the ordinal numbers forms.
In short, we also say „dwudziesta pierwsza hrywna”, but „dwadzieścia jeden hrywien”.
Should be like this: 21 ₴ (with a space). I must say, however, that even though it's been more than 10 years since it was introduced and you can easily type it, it's still not that widespread. If the government doesn't use it, who will? So far more often you'll see 21 грн.