The LanguageConverter::findVariantLink() method accepts two parameters about the link, a string for $link and an optional Title object.
The modified value of $link is from Title::getText(), which without the namespace prefix, uses spaces instead of underscores. However, there is no restriction on the input value.
The two parameters are redundant and can easily be misused (T328477), where the caller passes the DB key as the parameter and use the modified value as a DB key.
Also, it would allow undocumented use cases, e.g. with no Title object provided but with a namespace-prefixed link.

The new method to replace it should accept a PageIdentity or Title as the only parameter about the page to avoid ambiguity.