How do you use "sweet" or "sweetheart"?

Can I say "sweetheart" or "sweet" when I meet a kind person? For example, when my friend gave me some sweets, can I say "Oh, thank you! you're so sweet"?. Or when I talk about my friend to another friend, Can I say" She is a such a sweetheart"?
Sweet and sweetheart sound like a word that couples (lovers?) use.

In the UK, one would generally use 'sweetheart' to talk about someone of the opposite sex. The way you have used it in your example is accurate. You could also say that someone was 'my sweetheart', which would mean that they are your lover or partner.

I would say that you could say that someone of either sex was 'sweet'. In your example, the word is accurately used.

Happy Christmas!

Rob

What about my English language level

Hi, I have come here because I want to learn English language at a fluent level. I alway used to open a translator to verify what I wrote, but I don't want to do that anymore. In this text I did not open any translator.

I studied industrial engineering (comma deleted) at UdeA university, in Medellín, Colombia.
I am the chief commercial officer at S-SQUARE, a software development company.
I want to talk fluently, I want to watch movies, documentaries or any TV program with out subtiles and in English, not in Spanish, I want to read books with out a dictionary, and I want to listen music and understand what the singer says.

Ok, this one was my first post.
What do you think?

Hi Oscar.

Good work, you write well. As well as the corrections that I have made, I would suggest breaking up the last sentence of the second paragraph into two sentences. In spoken English it would be fine as it is, but it is a bit too long for a written text.

Happy Christmas!

Rob