There are multiple ways to identify a namespace in MediaWiki:

• Numerical namespace index, e.g. 0 for the main namespace, or 3 for the users talk namespace.
• Each namespace does have a unique, canonical English name. For example, searching for and linking to "User:<user name>" will work on all wikis in all languages, even if the namespace is localized. Example: http://de.wikipedia.org/wiki/User:Thiemo_Kreuz_(WMDE).
• Some namespaces do have additional aliases, e.g. "Image:" is identical to "File:".

Depending on the users experience (e.g. from https://www.mediawiki.org/wiki/Help:Magic_words#Namespaces_2), they are familiar with one or more of these identifiers, and might try to use them in Advanced-Search.

Expectations:

• Searching for numbers should work.
• Searching for canonical English names should work on all wikis, no matter what the wikis content language is.
• Searching for aliases should work.