Сейчас или никогда!

絶好のチャンスだよ。By my reckoning,
Russian = Now or never!
Japanese = It’s a great chance, opportunity! 絶好のチャンスだよ。
I can sort of see how these relate but it does not seem like the best translation. I was wondering if a native speaker, @MsFixer, might be able to help.

I looked up in the source. The Russian-Japanese pair is an indirect translation connected by Clozemaster. Indirect translations often cause “lost-in-translation” situations.

The English sentence “It’s now or never” was first translated in Japanese “絶好のチャンスだよ。” (ID: 141954) (lit. = “This is the best chance/opportunity.”) as a part of (notoriously know as) Tanaka Corpus. The EN-JP pair was imported to Tatoeba. Then, the Russian sentence was translated from English.

Tanaka Corpus (the original source) has another Japanese translation: “今しかない。” (ID: 172845) (lit. = “Now is the only right moment to execute it!”)

I definitely prefer ID: 172845 (今しかない) for the translation of the English phrase “now or never!”. ID: 141954 (絶好のチャンス) merely compares the current opportunity with ones available in the past. However, 絶好のチャンス doesn’t tell whether the better one would be available or not in the future. So, 絶好のチャンス misses the nuance of “or never”.

Yes, 今しかない, seems much better. 絶好のチャンスだよ almost seems like something you would say after “now or never” in certain situations. To me, in English, now or never, potentially implies an urgent decision, possibly involving life and death. ありがとう!