wagtail / 2 / contributing / general_guidelines.html

General coding guidelines

Language

British English is preferred for user-facing text; this text should also be marked for translation (using the django.utils.translation.gettext function and {% trans %} template tag, for example). However, identifiers within code should use American English if the British or international spelling would conflict with built-in language keywords; for example, CSS code should consistently use the spelling color to avoid inconsistencies like background-color: $colour-red.

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Licensed under the BSD License.
https://docs.wagtail.org/en/v2.16.3/contributing/general_guidelines.html