Good job with the sentences, they're all immediately understandable except for five which is a bit iffy. If you don't understand any of the corrections I made, let me know.

①When I went to the beach alone, I felt lonely because there were a lot of couples and families around me. 

②We feel like we don’t fit in our class. For example, when we do a presentation in front of everyone, they are suddenly quite "quiet" and look down. It seems like they can’t see us.

③What time is the concert going to start?

④My short term "goal" would be able to talk with the girls and have a girlfriend.

⑤I thought you spoke Japanese for being considerate of me because I didn’t understand your English.
⑤I thought you were considerate of me because I didn’t understand your English, so you spoke Japanese.

I thought it was considerate of you to speak Japanese because I didn't understand your English.

⑥Who’s the guy talking with you on Saturday? I wonder "if" he’s your boyfriend.

⑦I have some trouble with my English. I’m wondering "if" I would be able to understand English well in two months.

⑧Are you having fun? Yeah, I’m having fun!

⑨I have never been to ESL. I have taken "an" online English class before.

⑩You can't drink under 18 years old in your country, right? In my country, you can’t drink under 20 years old.