I say if you want a universal language everyone should learn Latin. Now you won't offend anyone. Except the French.
If you poll, you will find today that most people will not be aware of the concept of an international auxiliary language. This gives the adoption of one (not necessarily Esperanto) some kind of hope.
Latin even has a large amount of texts written at a basic level. Works like Summa Theologica and the Vulgate Bible were written in extremely simple language.
It has a very small vocabulary. There are some weird edge cases (imperative plural, for example), but even they follow common rules.