Add language slugs (mostly System namespace)

[gewarren]

Prep for dotnet/docs#45892.

(Hide whitespace changes.)

[learn-build-service-prod]

Learn Build status updates of commit 508ab26:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Text.RegularExpressions/MatchCollection.xml	✅Succeeded	View
xml/System.Text.RegularExpressions/Regex.xml	✅Succeeded	View
xml/System.Text/Encoding.xml	✅Succeeded	View
xml/System.Transactions/TransactionEventArgs.xml	✅Succeeded	View
xml/System/Byte.xml	✅Succeeded	View
xml/System/DateTime.xml	✅Succeeded	View
xml/System/Environment.xml	✅Succeeded	View
xml/System/Math.xml	✅Succeeded	View
xml/System/MathF.xml	✅Succeeded	View
xml/System/ObjectDisposedException.xml	✅Succeeded	View
xml/System/String.xml	✅Succeeded	View
xml/System/TimeZoneInfo.xml	✅Succeeded	View
xml/System/Type.xml	✅Succeeded	View

For more details, please refer to the build report.

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[BillWagner]

This LGTM @gewarren

Let's