I'd say: it depends on what application you have to build: 1) fat-client (OS specific) or 2) thin-client (Web).
When you decide to use second option, you can use c# within .net Core framework: Choose between .NET Core and .NET Framework for server apps | Microsoft Docs
In case of option one, you'll need to use Java or one of cross-platform scripting or interpreted languages. For further details, please see: Cross-platform software - Wikipedia
] is also open-source mobile app platform for .NET and C#.