We were impatient for the concert to begin.
Wrong tense. 待っている means “we ARE waiting”. The Japanese translation should be:
私たちはコンサートが始まるのを今か今かと待っていた。OR
私たちはコンサートが始まるのを今か今かと待ちわびていた。(more natural translation)
今か今かと is a typical reduplication to illustrate that the eagerness is restless. 今 = now, so 今か今かと means “please come now!”
待ちわびる (to be long for) is stronger than 待つ (to wait for the moment). So, 待ちわびる better captures the nuance of “impatient”.
We were impatient for the concert to begin.
You are right about the tense, of course, but wouldn’t it be better to report the sentence instead of commenting on it?
I checked the sentence source, and I see it is using the past tense in the Japanese translation now.
I have no interest in contributing to Tatoeba. I had terrible experiences with people, especially with admins and corpus maintainers who don’t speak Japanese properly but love to stick their noses into Japanese sentences. They just wasted my time.
I sometimes post alternative translations here on Clozemaster, wishing my explanations and example sentences help other learners. However, I will not do so on Tatoeba, and I request other Clozemaster users NOT to copy and paste them to Tatoeba without my consent.