Of the three you mention, French is by far the most important in Europe. Perhaps Mandarin in Australia because of proximity.
In Europe, Spanish is important only in Spain itself, and that is declining in favor of regional languages such as Catalan, Basque and Galician.
I hope you will not take offense, but your written English could be improved. I would work on that first, "I am a nurse planning on working abroad in Europe or Australia. which language is best to learn: French, Spanish or Mandarin ?" is standard English. I count eight small deviations. Your English is fine for social media, but for professional work as a nurse, a physician might find it unacceptable.