When performing a search, the language context of the query matters. If you're searching in en-CA, the search provider might only return results in that language, unless explicitly configured to include fallback.

You might need to manually include fallback languages in your search query. For example:

var searchLanguages = new[] { "en-CA", "en-GB" }; // include fallback
var results = searchClient
    .Search<YourContentType>()
    .Filter(x => x.Language.MatchAny(searchLanguages))
    .GetResult();

Are you using typed or unified search? 

Researched this further and Deepali's answer is correct. This approach works for both UnifiedSearch and typed searches.

I'd probably go with something like this

var langs = new[] { "en-CA", "en-GB"};
var results = _client
    .Search<IContent>()  // or your base type, e.g. SitePageData
    .For(query)
    .FilterForVisitor() 
    .Filter(x => x.Language.In(langs))
    .GetResult();