One string and one guard, from Paul's ruling on discussion 31: the UI is US English until a profile locale offers the choice.
What was wrong
The alignment buttons read "Align left / Align centre / Align right". The label was the only place in the whole feature that spelled it that way — align_center is the command, docs-align-center the class, and text-align: center the CSS keyword that makes it happen. Three names agreed and the one a person actually reads did not.
What changed
docs.editor.toolbar.align.center now reads "Align center". That is the entire product change; the catalog was scanned for every other British spelling and there were none in anything a user reads.
Two near-misses I checked and left alone:
internal/fonts' Licence() and its comment. British, but a Go identifier and a panic string, not copy — and the path it serves is already LICENSE-OFL-1.1.txt.fixtures/toolbar.mjs's comment citing align_center: 'centre' as the bug a guard once missed. That is an illustration of a wrong value, and it stays correct — arguably more legible now that the right value is unambiguous.
The guard
Nothing was checking which English the catalog was in, so the same drift could land again the moment someone adds a string. TestTheCatalogIsWrittenInUSEnglish walks every value and flags British forms against a word list.
A word list rather than a speller, deliberately: a dictionary is a dependency and a reliable source of false positives on product nouns, whereas the ~30 pairs a UK-based writer actually reaches for cost nothing and cannot misfire. Derived forms are listed rather than stemmed — a suffix rule for -ise turns "premise" into a finding, and a list that misses "colourless" until someone writes it is the better failure.
Tokenisation splits on non-letters, so "couldn't" becomes "couldn"/"t" rather than hiding a word glued to punctuation where an exact-match lookup would never see it.
Verified against the mistake, not just against itself — this project has found nine tests unable to fail, so the check is the point. The defect was put back, and the test failed naming both the key and the word:
render_test.go:133: docs.editor.toolbar.align.center: "centre" is British English — write "center"
Then removed again, and it passes.
Not this branch
Discussion 31: the locale on the profile. When en-GB exists as a second table, "Align centre" becomes its entry and this guard moves onto the en-US one. This branch is explicitly not a substitute for that and does not constrain its design.
Go suite green across all six packages.