Documentation

Every Markdown file in the repository is written in English. A repository has one working language, and documentation that drifts into another splits its readers: the next person to open the file may not be the one who wrote it.
This covers the root README.md, project documentation under docs/, guides, standards, and any instruction file an assistant reads.
Two things are not violations of it.
A file inside a locale folder is written in that locale's language. A path segment naming a language — es, en, pt — declares what the file holds, and a Spanish document under es/ is doing exactly its job. English remains the source.

handbook/onboarding.md              English, the source
handbook/es/onboarding.md           Spanish, translated from it

A document written for the client is written in the client's language. A feature analysis under an analysis/ folder is a deliverable someone outside the team reads and approves, so it follows that reader, not the repository. The same goes for any generated document handed to a user.
Everything else defaults to English, including a file that only the team will ever read. "Only I will read this" is how a repository ends up in two languages.
The validator checks each Markdown prose block independently. Outside the exceptions above, a block containing more than three distinct Spanish words is Spanish prose and is a violation, even when the rest of the document is English. Code fences and inline code are examples, not documentation prose, so they are excluded.
When a document has to change language, move it and translate it in separate changes. A diff that relocates and rewrites at once cannot be checked: a paragraph lost in the move looks exactly like a paragraph reworded in the translation.

A translated document and its source are one document in two languages, not two documents. Correcting a translation's wording is welcome: a clumsy sentence, a term nobody uses, a phrasing that reads as translated rather than written. Changing what it states is not.
Adding a step the English does not have, dropping a warning, softening a limitation, or updating a translated page while leaving its source behind all produce two documents that disagree, and a reader has no way to tell which one is true.
When the content itself has to change, change the English and translate it again. The source moves first, always.

Correct    — the English says the code arrives by SMS or by call;
             the Spanish is reworded so it reads naturally, saying the same thing
Incorrect  — the English says by SMS or by call;
             the Spanish adds "or by WhatsApp" because it happens to be true

Help documents carry a fingerprint of the English they were written from, so php artisan help:stamp --check can report a translation whose source has moved on.