Hello Pham, Andy,
Welcome to the Microsoft Q&A and thank you for posting your questions here.
I understand that you would like to know which NuGet package has ILanguageServiceClient
The ILanguageServiceClient is included in the NuGet package Microsoft.CodeAnalysis.Workspaces.
You can install it by installing Microsoft.CodeAnalysis.Workspaces package via Bash command using:
Install-Package Microsoft.CodeAnalysis.Workspaces
Or try to search for Microsoft.CodeAnalysis.Workspaces
in the NuGet Package Manager within Visual Studio. For more information on how to use it: https://www.nuget.org/packages/Microsoft.CodeAnalysis.Workspaces and https://learn.microsoft.com/en-us/dotnet/csharp/roslyn-sdk/
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I hope this is helpful! Do not hesitate to let me know if you have any other questions.
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Best Regards,
Sina Salam