The LaTeX scrlttr2 class is useful for using and re-using windowed envelopes. If the envelope is standard, the geometry may be known to the supplied KOMAscript machinery. If not, a few measurements can be given as parameters to align an address in a custom window.
To load the US №9 standard envelope, you would start with:
\documentclass[UScommercial9]{scrlttr2}
or for the French standard:
\documentclass[NF]{scrlttr2}
DIN is the token for the German standard.
If you reuse a non-standard windowed envelope, you can put the following in the preamble and tamper with the measurements as needed:
\makeatletter
\setplength{foldmarkhpos}{4.2mm} % default=3.5mm; distance from paper edge to fold mark; should account for the unprintable area of your printer
\setplength{tfoldmarkvpos}{108mm} % default=99mm; distance between top fold mark and top paper edge
\setplength{firstheadwidth}{190mm} % default=170mm for NF and \paperwidth for others; width of letterhead
\setplength{firstheadvpos}{10mm} % default=15mm for NF; distance from top edge to letterhead
\setplength{toaddrvpos}{40mm} % default=35mm; distance between top of window and top paper edge
\setplength{toaddrhpos}{98mm} % default=-10mm; distance from the left edge of the paper to the address field (if positive)
\setplength{toaddrindent}{5mm} % default=10mm; left and right indentation of the address within the to-address box
\setplength{toaddrheight}{40mm} % default=45mm
\makeatother
UPDATE:
Word of caution: someone in Mastodon once posted an envelope they received in the UK from their hospital that had a quite big window, and at the bottom of the window under his address was “the results from your colonoscopy…” in full view of anyone handling the envelope.
So indeed, privacy snafus happen. LaTeX’s default and normal settings never result in that kind of leak, but if you do a lot of manipulation to fit a lot of text on the first page, that sort of exposure is within the realm of possibility.