1. No worries - Only works for Aussies. SoCal folk have attempted to co-opt this but it's just not taking.
2. Cheers - Solely for Brits. Non-Brits attempting to sound worldly just come off as desperate Anglophiles when they use this.
3. Hola - If you're not a native Spanish speaker, stop kidding yourself. Same with 'no problemo.'
4. Gesundheit - Just go with 'bless you' unless you're German. Even if you're an atheist. The words have no meaning so you're not undermining your beliefs.
5. Aboot - It sounds adorable when Canadians say it. Out of anyone else's mouth it comes off condescendingly.