This is not only possible, but pretty standard.

        if (!examineManager.TryGetIndex("ExternalIndex", out var index))
        {
            // ...
        }
        var searcher = index.Searcher;
        var query = searcher.CreateQuery("content"); 
        var currentCulture = CurrentThread.CurrentCulture.Name.ToLower();
        var searchFields = "pagetitle_{0},[...]";
        var terms = query.GroupedOr(string.Format(searchFields, currentCulture).Split(','), searchTerm);
        terms.Execute();

Please note that the External Index only contains nodes that are published to all languages. There is some reasoning behind this that I don't fully follow. And it can probably be remedied by configuring the indexer.