In order to translate you need:

• folder called i18n
• file module_name.pot within i18n (example: mymodule.pot)
• file language.po (or languate_country.po) within i18n (example nl.po or nl_BE.po)

For a report, you need lines within the .pot:

#: report:sale.order:0
msgid "Offer"
msgstr ""

within the .po:

#: report:sale.order:0
msgid "Offer"
msgstr "Offerte"

The issues I have run into are:

• updating the .pot and .po files does not always show on the reports, if these have been stored. So you need to create a new situation and print the report for the first time.
• When you miss an entry in your .po file, there might be a default entry in the database, and that one will be used, even if you update your .po file.
• it does matter where you place your :. A text like "the answer:" will not be translated it you store "the answer" or "the answer :" in your .pot and .po files.