Download and install the version for your desired language. https://www.thunderbird.net/thunderbird/all/

There is no need to uninstall, just run the downloaded installer.

Apparently it is a bug as all four of my Linux PCs have British spellings in the Thunderbird UI when US English was selected at install. Clearly some spelling differences were missed. When I posted I thought it was an anomaly of that one PC but it isn't.

You can follow the suggestion or not. Up to you.

The point is that I did that, yet Centre remains.

Please post a screenshot illustrating the problem. https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

Couldn't do a screenshot as the selector disappears from it so I had to take a photo of the screen instead.

The fact is, the language of the interface, menus etc. is separate from the language of the spellcheck dictionaries. You can have the EN-US spellcheck in the EN-UK version of TB, or the EN-UK spellcheck in the EN-US version - or have both dictionaries in either version.

For TB 60.7, the correct webExt dictionaries are:

US English

UK English See instructions on this page for installing dictionaries.

Yes, I know that and installed with US English as the UI choice, and hence the question. All four of my Linux systems were set up and configured with Thunderbird's US English yet all have Centre rather than Center and maybe a few others.

I'm running TB 60.7.0 en-US on Linux here too, and for me it shows 'Center'.

What finally did the trick was to decompress the downloaded .tar.bz2 and forceably copy the contents over the current install. Once that was done, it now shows Center for me too. Not sure why four out of four PCs had Centre and remained that way through all updates but now at least there is a fix even if not too elegant.