There are several special namespaces in Wikidata, and none of them is conveniently translatable. What's worse, each of them is hard to translate for different reasons.
Item and Property
In English, the namespace tab in the main space is shown as "Item". That's because nstab-main was manually set to "Item" in the MediaWiki space.
This isn't good. In most other languages, the tab is called "Page", the default translation of nstab-main. In a few languages, it was translated by administrators, but it's not the right way to do it.
All the same is true for the Property space.
Both namespaces and their associated talk spaces can be translated in repo/Wikibase.i18n.namespaces.php. They are translated there to Hebrew, for example, and the names could be used as tab titles, but they aren't.
Perhaps it would be better to make it translatable in translatewiki, because translating this file requires using Git, which most translators are not going to do. But at least using the file would be better than translating in the MediaWiki on Wikidata.org.
EntitySchema
This extension doesn't even have an .i18n.namespaces file. The tab title is translated to some languages locally, but it's bad.
The names for the EntitySchema and EntitySchema talk spaces appear to be defined in extension.json, only in English. Adding a EntitySchema.i18n.namespaces.php file, and actually using the translations there or making nstab-entityschema and nstab-entityschema_talk would be great.
Lexeme
The Lexeme namespace name appears to be hard-coded in src/WikibaseLexemeHooks.php, and not easily translatable in a PHP file or in translatewiki. The tab title is translated to some languages locally, but it's bad.
A solution similar to what is suggested for EntitySchema would be good.