Don't worry about that. "Tomáš" and "Tom%C3%A1%C5%A1" are technically the same thing (opi.wiki/item/Tom%C3%A1%C5%A1_Plekanec). What needs to be done is improving our search engine (not only because of this issue, but in general).
What should be done with this?: opi.wiki/item/Tomáš_Plekanec
Wikipedia's article is en.wikipedia.org/wiki/Tom%C3%A1%C5%A1_Plekanec
On Wikipedia, if you type Tomas Plekanec instead of Tomáš Plekanec, it will redirect to the Tomáš Plekanec article.
If I type and search for Tomas Plekanec on OpiWiki, the item won't appear. There's no redirection. It's as if the item doesn't exist. If I type Tomáš Plekanec, I'll find the item.
I guess I should've named the item Tomas Plekanec. I just want to make sure I got this right before I create more items with funny characters.
I just had a quick look at the guidelines and nothing seems to be mentioning this issue.