The advanced search values are based on the source code value types. If "Language" is represented as an int, then you can search using a number.
As already noted by @theSLAYER, most cards do not specify a language; when the gift is received by the delivery man, they will inherit the save file language.
There's no computerized database of what cards can be received by a language / region... It's one of PKHeX's unaddressed issues (#275)