Thanks for asking this question, deciding what names to use for pre-1500s profiles can be a difficult process.
Although the EuroAristo naming guidelines are the main standard for all pre-1600 profiles http://www.wikitree.com/wiki/Name_Fields_for_European_Aristocrats we also have to keep in mind the overall naming standards for Wikitree.
The Proper First Name and Last Name at Birth fields should be the names they were born with, in their native language. The Preferred First Name and Current Last Name should be the names they used at the end of their life.
So if we are discussing English people who were born in England, but perhaps inherited land in Europe or vice versa, then that would dictate what names to use and in which language.
If you wanted to note an alternative name in another language because they are famous in that country but were born and died in England for instance, then you might decide to record that name in the Other Nicknames or Other Last Name/s fields.
It can get quite complicated and the best option if you're not sure is to ask (it's often easy to discuss specific cases).
You will also find that there are examples of profiles on Wikitree that don't follow these guidelines, perhaps left over from times when a gedcom could be downloaded, no questions asked.