今日、日本語だけで会話しちゃってごめんね。って英語でなんて言うの？

"I'm sorry for...." & "I apologize for..." are phrases that can be used to apologize for something that you did or that someone else thought you have done.

When you are part of a group of people who are a different nationality to yourself and they are speaking their native language it can make one feel isolated or left out of the conversation.
If you are one of the group who are speaking then when apologising it is polite to first of all apologise and then explain that you hope they did not feel 'left out' of the conversation. Some people enjoy listening to others speak their native language as this is a good way of learning new phrases or words.

My Swedish friends used to speak Swedish a lot around me and after they had finished their conversation they would often say, "sorry for the Swedish" - but meaning the Swedish language not Swedish people. It's a polite phrase to make you feel not so left out and forgotten!

The two sentences provided above are great ways to apologize to someone who did not understand Japanese. In the second sentence you will see the verb gab. This word means to talk or chat. We use this word in our everyday conversation, so it would be appropriate in an informal setting like talking with family or friends.

If you apologise you are saying you are sorry for something so to say "we apologise/are sorry for only talking in Japanese today" that gets the point across to the person easily

When you want to apologize to your Taiwanese friends that you only spoke Japanese in your conversation with them; then you can say:
-I'm sorry, I don't understand Taiwanese. I;m sorry for only speaking Japanese to you
-I don't speak Taiwanese, that's why I speak Japanese to you all the time.
-I'm sorry we only spoke in Japanese