Currently on non-English projects, you can't use all: syntax (all: in the beginning of a query or prefix:all: in the ending) to search across all namespaces. Instead, you can use its translated versions, such as: все: on ruwiki, alle: on dewiki etc. Given that all other keywords are English-language and, for ruwiki, with prefix:все: you have to switch keyboard layout, it's unuseful and counterintuitive.
Step to reproduce: follow the links:
Interestingly, all: syntax is mentioned on ruwiki search help page, so probably it worked some time ago.