It depends on the setting of your story.
If set in the real world, it's best to conform to the usages of either the country your story is set in, or, if writing from the point of view of a character, using what would seem natural to the character.
You can even play the differences for laughs, e.g. the American/British tourist using the dictionary/guide book and getting the wrong translation and confusing the French/Italian/Russian or whatever native or getting laughed at.
Or use it as a plot device with someone killing the wrong person because of the differing floor numbering systems.
In a fictional world, pick a style you're comfortable and stick with it. Consistency is less confusing to the reader.
I would say UK spelling, grammar and usage is more commonly used internationally than US, especially among second language users, but each "English" speaking country has its own peculiarities.
Also, while the rest of the world likes to make fun of the US when it comes to speaking/writing "English" most people have seen enough US produced TV shows and movies to at least understand it, so if you are more comfortable with US usages, by all means use that.
About the only thing that may require clarification that can't be inferred from context is floor level in a multi-story building, where UK (and many other countries) use the "Ground Floor" designation for what would be numbered 1st Floor in the US and start numbering (1st floor) from what would be the 2nd floor in the US.
Things like horn/hooter, hood/bonnet, elevator/lift, apartment/flat, etc. are rarely confusing, as it can be figured out from the context.
Ooh ... Just don't use "pissed" without clarifying. Apart from when used as a verb to mean urination, when you say someone "is pissed" in the US it means they are angry, while in the UK it means they are drunk.