English is checked in the variant your document is written in, and other languages follow their regional conventions.
How the variant is chosen
Silvertext reads the spelling and punctuation already in the document. A draft with "colour" and "organise" is checked as British English and "color" is not flagged as an error; a draft in American spelling is the reverse. When a document mixes the two, the majority wins and the minority spellings get cards.
Setting a preference
In Settings you can set your writing region. The report then uses that variant's spelling, quote marks and punctuation placement for its fixes and citations, even when the document itself is short or ambiguous.
Citations follow the region too
The quote marks written around a cited passage, and whether a comma sits inside or outside them, follow the language and region of the passage, so a French quotation in an English paper keeps its guillemets.
Other languages
Spanish, French, German, Portuguese, Italian and Dutch are checked with their own conventions. Regional variants inside those languages (Brazilian and European Portuguese, for example) follow the document where the difference is detectable.