The official language of Canonical documentation is English, or en-GB to be more precise. There are all sorts of minor differences between English and American English, including spelling, verb morphology, transitives, etc.
Many of these differences will thankfully have no impact on writing the documentation for MAAS though - it is unlikely you will be talking about “pants” or “hockey” or “tabling motions” for example. The main and most notable difference is in spelling.
Popular belief is that you will merely need to add a few 'u’s to words and change -ize to -ise everywhere. It is a bit more complicated than that though. In fact, many -ize endings are acceptable in en-GB, though the -ise endings are generally preferred. The Oxford English Dictionary Style Guide (1998) has this to say:
“The verbal ending -ize has been in general use since the 16th century; it is favoured in American English and in much British writing, and remains the current preferred style of Oxford University Press in academic and general books published in Britain. However, the alternative spelling -ise is now widespread (partly under the influence of French), especially in Britain, and may be adopted provided that its use is consistent. … A number of verbs always end in -ise in British use, notably advertise, chastise, despise, disguise, franchise, merchandise, surmise, and all verbs ending in -cise, -prise, -vise (including comprise, excise, prise (open), supervise, surprise, televise, etc.), but -ize is always used for prize (=value), capsize, size. … Spellings with -yze (paralyze, analyze) are only acceptable in American use.”
As you can see, it can be rather tricky. And that is only the ize/ize issue. There are many other endings which differ (e.g. -eled/-elled as in “travelled” and “labelled”. The best way to ensure that you are using consistent en-GB spelling is to simply enable the en-GB dictionary on whatever software you generally use for writing. For example, in
vim you could execute the command:
:setlocal spell spelllang=en_gb
This will change the spelling highlight options for the local buffer only, so you won’t have to worry about whatever language you normally use. Do not worry about your atrocious grammar (in either variant of the language) as the developer advocates are used to tidying that up!