Я бачу гарні дівчата з квітом у волоссі! (Thinking about Ukrainian folk singers here.)
Я бачу гарних (accusative plural) дівчат (accusative plural) з квітами (instrumental plural) у волоссі. But good try! Any Ukrainian would understand what you wanted to say.
Just wondering. Do all feminine nouns in the acc. pl. lose their -a? Because 'Я люблю мов' just sounds weird to me. Also do you know how I could get back into doing Ukrainian? I finished the course and I liked Ukrainian but now it's kind of boring going back and strengthening skills. My grammar is alright but definitely needs work and I don't remember many words
You are right "дівчина" and "мова" belong to the same group of nouns, but "дівчина" is an exception and it is declined like neuter nouns (from the fourth declension). This is because its plural "дівчата" coincides with the plural of the neuter noun "дівча" (little girl). "Мова" is declined normally and would be "мови" in the accusative plural.
As to getting back into doing Ukrainian. You could do the reverse tree (English from Ukrainian). You could further develop your vocabulary by doing Memrise courses (Duo, the complete list). You could develop your comprehension skills by watching Ukrainian TV online. You could dig into grammar. You could find a conversation partner (for example, in the English from Ukrainian discussion). You can look through Top All Time discussion for more ideas.
How do you know if a word like мова is a part of the first declension and not the second? Declensions and some other things like prepositions, aspects, declining numbers, adjectives, etc. is where I left off with Ukrainian
I really think I should take a class on Ukrainian in the future as even though I've kinda stopped learning it, it still interests me
To tell the declension of a noun you have to know its gender and then you look at the ending (I know, remembering genders is difficult enough already...). This and other topics you mentioned are probably the hardest. My way is to learn and understand such topics once and then have a lot of practice. At some point you start feeling what is right or wrong (as you did with 'Я люблю мов', and like all natives do).