The search query alter is not respecting its own language filter. Instead it uses the current language to create the solr ID of an entity for a boost or exclude. A solr query is not necessarily querying using the current language, this could be a fixed language as well (or 'und').
Create entities with no specified language (und).
Create a search page (View) displaying entities in English and undefined language.
Create a search override excluding one of the undefined langyage entities above.
Visit the search page in English and the entity will not be excluded. The solr entity ID will contain the current language 'en' while it's only known in Solr with language 'und'.
In the query_alter hook, use the query language(s) to generate the solr ID instead of the current language.
Adjust the query_alter hook as described above.
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Needs review
2.1
Code
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