Learn English – This issue should be resolved now or it should have been resolved now

The meaning is ambiguous, but here is one possibility:

Had this been a solution that could be provided in Java 5 and above, the overloaded "replace(charSequence, charSequence)" could have been used.

The tenses are inconsistent in the first example, and "Have this been..." in the second example is not normally used.

Actually I'm a writer. The difference is enormous. You probably cannot write a single sentence that would please a rigorous editor in the U.S. and a similarly qualified person in the U.K.!

It is not just a question of spelling a few words. The differences are so pervasive that I personally as a professional U.S. writer would be incapable of writing text which would not be massively re-written if I submitted it to a British editor.

So yes, you have to choose. Questions to consider, in order of importance.

1. Which are you most comfortable in already? Your text above read nicely to me, so I suspect perhaps you are most comfortable in American English.

2. Which English does your editor speak? If you write your text in perfect American English and your editor has learned British English, it's going to be a long, hard edit.

3. Which type of English are your source materials written in? It's going to be a lot easier to write American English if all the texts you are drawing from are in American English. And of course the reverse is true!

I suggest relying on these factors since it's impossible to know whether your audience prefers British or American English. Britain is closer geographically for Europeans, and BE is the language often used in airports, hotels and train stations. However, many Europeans have spent time in the U.S., or at least watch American movies. In the scientific realm, there are more publications written in American English so scientists may have more exposure to it.