KDE address a world wide audience and is therefore fully localized. Localization means to adopt language, text layout, numerals as well as cultural aspects and regulatory compliance.
Provide bidirectional mode dependent to language (left-to-right vs. right-to-left).
Make sure your GUI displays correctly in languages written from right-to-left
Allow translation with [http://www.gnu.org/software/gettext/manual/html_mono/gettext.html#Plural-forms| multiple plural forms].
Design GUI with enough room for long captions.
Respect system-wide locale settings (unit systems, date and time format, currencies etc.).
Take cultural comprehension of icons (Red Cross vs. Red Crescent) into account.
Do not localize standard short-cuts.
Be aware of intercultural (mis-)comprehension of acronyms.