I logged in to zh.wikipedia.org and it says I am $1.
您現在以「$1」的身分登入維基百科。
Pure MediaWiki doesn't act that way. It just jumps to the homepage. So must be a wikimedia bug.
en.wikipedia.org doesn't have that problem.
Version: unspecified
Severity: enhancement
Not fixed. Below is what I just pasted from Firefox.
In Preferences I had
介面語言: zh-hant
字體變換: zh-tw
不轉換連結標題 checked
不進行用字轉換 checked
登入成功
維基百科，自由的百科全書
跳轉到: 導航, 搜索
您現在以「$1」的身分登入維基百科。
如果您對維基百科有任何問題的話，請參看幫助頁。
您已登入其他的維基媒體計劃：
meta.wikimedia.org.wiktionary.org.wikibooks.org.wikiquote.org.wikisource.orgcommons.wikimedia.org.wikinews.org.wikiversity.org.mediawiki.orgspecies.wikimedia.org
返回到Wikipedia:首页。
Ah ha, unchecking those two choices, but mainly making both languages = "zh" fixed the problem. So there you go, there must be a missing translation string for zh-tw.
Checked Disable link title conversion and Disable variants conversion, chose zh-hant as language and zh-tw as variant. Still can't reproduce it.