You can directly condition language using the below substitution usages.
[cb:if language_code="en"]English Only[/cb:if]
[cb:if language_tag="en-GB"]English Only[/cb:if]
I'd probably do something like the following.
Field: Custom > Value
Value: [cb:if language_code="en"]allowed[/cb:if]
Translate Value: No
Operator: Equal To
Value: allowed
Translate Value: No
Mode: Show
Added a feature ticket to review adding a custom condition for language to make it easier.
forge.joomlapolis.com/issues/6241