[BUG] Spell checker uses US English despite system language set to English (United Kingdom)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

The Claude desktop app (Windows) flags correct UK English spellings (e.g. "summarise", "organise") as spelling errors. The Windows system language is fully configured as English (United Kingdom) - it is set as the Windows display language, the primary preferred language, and the regional format. English (United States) is also installed but listed second.

Steps to reproduce:

1. Set Windows language to English (United Kingdom) as primary (English (United States) may also be installed but listed second)
2. 2. Open Claude desktop app
3. 3. Type a word with a UK English -ise suffix (e.g. "summarise") in the message input
4. 4. Observe red underline indicating a spelling error

Expected behaviour:
UK English spellings should be accepted when the system language is configured as English (United Kingdom).

Actual behaviour:
The spell checker rejects valid UK English spellings, suggesting it is using a US English dictionary regardless of system language settings.

Environment:

• OS: Windows 11 Enterprise (10.0.26200)
• - System display language: English (United Kingdom)
• - - Primary preferred language: English (United Kingdom)
• - - - App: Claude desktop

What Should Happen?

aUK English spellings (e.g. "summarise", "organise", "behaviour") should be accepted without spell check errors when the Windows system language is configured as English (United Kingdom).

Error Messages/Logs

Steps to Reproduce

1. Set Windows language to English (United Kingdom) as primary preferred language (English (United States) may also be installed but listed second)
2. Open Claude desktop app
3. 3. Type a word with a UK English -ise suffix (e.g. "summarise") in the message input
4. 4. Observe red underline indicating a spelling error

Claude Model

None

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.139 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_