Update Language.fr-FR.resx and added french to FlexibleMessageBox.cs - #476

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: microsoft/setup-msbuild@v2, nuget/setup-nuget@v2. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/