WPF TextBox should respect unicode Language Tags during spellcheck to enable spellcheck of multiple languages in the same sentence.

60-75 percent of the world population uses multiple languages, the TextBox spellcheck should work when anyone mixes languages to express themselves in the way that works best for them rather than restricting them to a single language.

There are many scenarios where users communicate in multiple languages in the same sentence, but the WPF textbox will only spellcheck one language marking the other language text as misspellings.

The RichTextBox does handle multiple languages with by setting the language on separate runs. But, this is not appropriate when other systems require plain text.

Unicode does have language Tags and these could be used during spellcheck to improve the experience.
or just allow setting multiple languages for spellcheck on the control.

This will enable foreign doctors to write patient notes in their local language and the medical terminology in English.
Enable discourse using mixed language between users that are fluent is different languages.
Or languages/cultures that use English (or other language) for new foreign words.
and many other scenarios.