Your buyers are all over the world, and a great many will need your item in another language. You do not have to speak it. You have to make it possible.
Get the strings out of the code first
Every visible piece of text in one place, referenced by key. This is the whole job. A buyer can translate a language file in an evening; nobody will go through four hundred templates hunting for hard-coded words.
Do not build sentences from fragments
Concatenating "You have" plus a number plus "items left" makes a sentence that cannot be translated, because word order differs. Keep the whole sentence as one string with a placeholder in it.
Leave room
German and French run considerably longer than English. A button sized exactly to its English label breaks in half the languages your buyers use. Let layouts grow.
Dates, numbers and currency
Do not hard-code a format. 03/04 is two different days depending on where the reader is, and a decimal point is a comma across most of Europe. Use the platform's formatting with a locale rather than string building.
Right to left, if you can
Arabic, Hebrew, Persian and Urdu read right to left, and a layout that assumes otherwise is unusable rather than merely awkward. Supporting it is mostly using logical CSS properties rather than left and right.
Say so on the item page
"Translation ready, language files included" is a selling point for a large share of buyers, and most listings never mention it.
