404 page breaks multi-language switcher

[welpo]

Bug Report

Environment

Zola version: 0.18.0
tabi version or commit: latest

Expected Behavior

On the 404 page, the language switcher should take the user to the main page of each language.

Current Behavior

Changing the language takes the user to a broken, almost-correct URL. For example, to go to the Spanish demo site, it should lead to:

https://welpo.github.io/tabi/es/

But instead leads to:

https://welpo.github.io/tabi/es /

In short: there's an extra space after the language code.

Step to Reproduce

Visit the 404 page and change the language through the top right globe icon.