I have guests from other another country visiting on next Friday!! 

It is my pleasure to meet them but I`m a little bit nerves nervous.
I have a an opportunity to do a short 15 minute presentation 15min about roof gardens.
They are My guests include a professor and his students. I want to talk with them about greenery.

I hope they could will enjoy this meeting indeed. But, if could I speak well in English with them....?

You are to use "another" and not "other" here. This word is very difficult to explain in this context. It essentially means "an other" but the two words come together to form "another". So, essentially, it means "They come from an other country" or "a different country".

Always use "an" before vowels (a, e, i, o, u) and a before consonsants (the rest of the alphabet).

Your final sentence is a bit confusing. I have noticed a lot of Japanese people finishing their notebooks with open-ended statements, but maybe they don't translate all that clearly. I think you are trying to say: It would be great if I could speak well in English with them.