Country and stationery variations

Set paper size, margins, header/address bounds, typography and assets for each letter through builder methods or template attributes.

Configure a layout before rendering the body:

var letter = Letter.builder()
    .page(PageSize.LETTER)
    .margins(new Margins(15, 18, 17, 22))
    .header(recipientAddress)
    .headerBoundsMm(88, 23, 78, 28)
    .body(bodyText)
    .build();

Those measurements are illustrative, not a claim of conformance to a postal or national letter standard. Measure the actual envelope/window/stationery, convert paper-edge coordinates using header placement, and inspect a printed sample at 100% scale. Printer scaling and physical tolerances are outside PDF generation.

With HTML templates, use scalar geometry placeholders:

<letter page="${paper}" margins-mm="${margin}">
  <header x-mm="${addressX}" y-mm="${addressY}"
          width-mm="${addressWidth}" height-mm="${addressHeight}">
    <p>${recipientName}<br>${street}<br>${postalAddress}</p>
  </header>
  <main><p>${message}</p></main>
</letter>

Supply those keys along with recipient data in .values(...). Use separate placeholders for address lines because template whitespace follows HTML rules. Use explicit LocaleSettings with param for currency, dates and decimal separators; application text such as salutations remains caller-supplied. Choose/register fonts that cover the actual language.