TL:DR: Spellcheck suggestions for OS languages are broken in latest Steam beta.
Only internal Chromium dictionaries work.

By default, Steam uses the primary OS language for:
- the Steam Client UI
- CEF (always fixed, should align with the Steam Client UI instead!)
- spellchecking suggestions

This issue is about the spellchecking suggestions:
Steam / Chromium can use the spellchecking dictionaries provided by the operating system (e.g. Windows).

In Steam beta version 1760657741 (which uses Chromium: 126.0.6478.183) the spellchecking suggestions no longer appear if they are provided by Windows.

If I add "English (United States)" to the language list in Windows, the spellcheck stops working in Steam for english words (no suggestions in context menu),
because it tries to use the Windows dictionary instead of the Chromium dictionary.

This also means that if you set a spellchecking language in Steam which does not exist in your OS (e.g. "Spanish (Spain)") then the spellcheck suggestions do appear for that language in the context menu.

Also, only for non-OS languages does Steam / Chromium actually download the bdic file into (the dictionary for spellcheck corrections / suggestions).

Windows 10
Steam Version: 1760657741 (Chromium: 126.0.6478.183)

• Install the latest Steam beta
  
• Mistype words in your primary OS language in Steam
  
• Right-click on the underlined words

No spellcheck suggestions appear in the context menu.
The only option is "Add to dictionary".

Spellcheck suggestions should appear correctly.

(Also, please make the spellcheck context menu clickable again.
It's been months since "Add to dictionary" broke.)