... will use any characters outside the ASCII range
do you use UTF-8 (Unicode character encoding) throughout? Importantly, this means for all your back-end processes as well as for your user interface.
Characters
... contains natural language text that will be read by a human (this includes error messages or other UI text, JSON strings, etc, etc),
does it correctly label content for language?
does it correctly label content for text direction, and utilize techniques that help localize for bidirectional Asian and African writing directions (eg. logical CSS properties, dir=auto on forms, etc.)?
does your styling allow for the different typographic approaches used around the world (for things such as line-breaking, text justification, emphasis or other text decorations, text selection and units, etc.)?
have you cleanly separated content and styling?
Language
Text direction
... allows searching or matching of text, including syntax and identifiers
have you understood the implications of normalization, case folding, etc?
Handling character normalization
... sorts text
does it do so in locally relevant ways?
... captures user input , including names, addresses, time & date formats , etc
are your user interface forms flexible enough to cope with wide variations in data formats and presentation? And can you capture all necessary data in the back-end databases, at an appropriate level of detail?
do your forms capture metadata about language and text direction, and accommodate locale-specific input methods?
Working with personal names
Working with date formats
Managing text direction in form controls
... involves working with different currencies
do you know how to set prices that reflect the varying socio-economic levels of your customers?
do you have a mechanism in place for handling currency conversions for subscriptions and payments?
Local dates, times and formats
... deals with time in any way that will be read by humans and/or crosses time zone boundaries
does it represent time as expected in locales around the world, and manage the relationship between local and global/absolute time?
Local dates, times and formats
... will be translated or localized
do you use content negotiation correctly, and follow appropriate guidelines on navigating between language versions?
can it be adapted to suit different cultural norms around the world (ranging from depictions of people or gestures, or relevant examples, to expectations about gender roles, to approaches to work and life, etc)?
do you have a plan for how to display data in local formats, addressing things such as names, addresses, numbers, dates, currencies, etc.?
Navigation
... will be rolled out in other countries
are external system dependencies available to your user, and in a language that is appropriate for your user interface?
will the population you want to reach have access to sufficient bandwidth (or even the internet) when they need to use your application?
did you take into account that a large majority of users these days interface with the Web via mobile devices?
have you clarified whether the community you are targeting one that is familiar with credit card transactions or online payments?