Shared language does not mean that a successful localization to one territory will work in a different country with the same language.
There are cultural differences that can be very difficult to predict.
For example, the color orange may not have strong connotations to most Westerners, but in Ireland its association with Protestantism is still widely recognized.
In user testing, we have seen South American users complain that images on a large international website were “US-biased.”