Add unit test for this case.

src/EditorFeatures/Test2/EditorServicesTest2.vbproj

@@ -249,6 +249,7 @@
     <Compile Include="IntelliSense\CompletionRulesTests.vb" />
     <Compile Include="LanguageServices\SyntaxFactsServiceTests.vb" />
     <Compile Include="NamingStyles\NamingStylesTests.vb" />
+    <Compile Include="ReferenceHighlighting\DocumentHighlightsServiceTests.vb" />
     <Compile Include="Simplification\BlockSimplificationTests.vb" />
     <Compile Include="Simplification\SimplifierAPITests.vb" />
     <Compile Include="CodeFixes\CodeFixServiceTests.vb" />

src/EditorFeatures/Test2/ReferenceHighlighting/DocumentHighlightsServiceTests.vb

+Imports System.Collections.Immutable
+Imports System.Threading
+Imports System.Threading.Tasks
+Imports Microsoft.CodeAnalysis.Editor.UnitTests.Workspaces
+
+Namespace Microsoft.CodeAnalysis.Editor.UnitTests.ReferenceHighlighting
+    Public Class DocumentHighlightsServiceTests
+
+        <WorkItem(441151, "https://devdiv.visualstudio.com/DevDiv/_workitems/edit/441151")>
+        <Fact, Trait(Traits.Feature, Traits.Features.ReferenceHighlighting)>
+        Public Async Function TestMultipleLanguagesPassedToAPI() As Task
+            Dim workspaceElement =
+                <Workspace>
+                    <Project Language="C#" CommonReferences="true">
+                        <Document>
+                            class C
+                            {
+                                $$string Blah()
+                                {
+                                    return null;
+                                }
+                            }
+                        </Document>
+                    </Project>
+                    <Project Language="Visual Basic">
+                        <Document>
+                            Class VBClass
+                            End Class
+                        </Document>
+                    </Project>
+                </Workspace>
+            Using workspace = TestWorkspace.Create(workspaceElement)
+                Dim position = workspace.DocumentWithCursor.CursorPosition.Value
+
+                Dim solution = workspace.CurrentSolution
+                Dim csharpDocument = solution.Projects.Single(Function(p) p.Language = LanguageNames.CSharp).Documents.Single()
+                Dim vbDocument = solution.Projects.Single(Function(p) p.Language = LanguageNames.VisualBasic).Documents.Single()
+
+                Dim service = csharpDocument.GetLanguageService(Of Microsoft.CodeAnalysis.DocumentHighlighting.IDocumentHighlightsService)
+                Await service.GetDocumentHighlightsAsync(
+                    csharpDocument, position, ImmutableHashSet.Create(csharpDocument, vbDocument), CancellationToken.None)
+            End Using
+        End Function
+    End Class
+End Namespace
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