The Prizm OS supports a few localization features. Much of the UI can be shown in a language other than English. The OS has six built-in languages for messages (English, Spanish, German, French, Portuguese and Chinese), and two built-in languages for the function key labels (English and Chinese). It is prepared to support a probably unlimited number of other languages for messages through language add-ins (see CASIOWIN#Add-in software and language and Versions and versioning#Official add-in languages), and it probably also supports add-in languages for function key labels. Function key labels consist of bitmaps - there is no text involved, even for labels which only display text.
The user can select a language for messages independently of the language chosen for function key labels.
At a basic level, translation works by assigning an ID to each string used in the OS, then translating each string separately. The translation will appear everywhere the string with that ID is used. Supposing the string with ID 123 is "Hello World", and a message language translates ID 123 to "Foo Bar", then every place where string with ID 123 appears, will show as "Foo Bar".
The settings for number and currency formats appear to be completely disconnected from the currently selected languages, and there is no way to change decimal separators and the like. There appears to be no support for different collation settings (this would be mostly visible in the way items are sorted in the file browsers). Common words are not ignored when sorting. Message translation may take into account some culture aspects, but none of it is automated - things like abbreviations and numerals must be explicitly specifying them in the translated messages.
Locale in add-in software
Add-ins can display messages and function key labels from the OS library of messages, but they can't register their own messages or labels. For messages and labels not included in the OS, or where not all translations apply (depending on context), add-ins must provide their own localization system. This is what official add-ins appear to do.
Add-in languages are distributed as g3l files, and are meant to be put in the root of the storage memory, just like add-in software (g3a files). As far as it is known, each g3l file can only provide one additional language. These files share the common Casio header. The rest of the file has the same structure as the message tables included with the OS.
Once a valid g3l file is installed, its language becomes available for selecting on the Language menu of the built-in System app. When an add-in language for messages is selected on that menu, a section of its g3l file is copied into Flash at address 0x80B60000 / 0xA0B60000, effectively creating a patch that looks very similar to what can be seen in the areas where messages for built-in languages are stored. The OS then refers to this memory area when looking up a certain message. Presumably, the OS keeps somewhere a pointer to the start of the currently selected message table, and changing this pointer is what effectively changes the currently selected message table.
Apparently, if for some reason the OS finds a problem with the installed add-in language, it will automatically revert the language message setting to English.
Using the built-in messages
Some syscalls will allow you to print localized messages on screen, or copy them to buffers, given their ID (a positive integer). A list of such syscalls is available here. So far, the IDs for messages have been consistent across OS versions, something necessary to keep compatibility with existing add-ins. Messages added with each OS version receive IDs higher than the previous version's highest ID. However, existing messages may change (because of e.g. better translation) to something which has the same meaning in the contexts where Casio has used it so far, but obviously they do not care about, nor have a way to track, how the messages are used in unofficial add-ins, so these changes could break your add-ins even if just in aesthetic terms.
Each message may be up to 88 bytes long. If storing messages on a buffer (when using LocalizeMessage1, for example), make sure your buffer is big enough. For safety, you should still provide a big enough buffer even if you're sure the message is much smaller, because one can never be sure of how syscalls decide to work with the buffers one passes to them. Also note that in the case of languages like Chinese or Russian, messages take many more bytes than they appear to take and sometimes much more than their English versions, partly because of how multi-byte encoding works.
Messages in Chinese are meant to be printed using a special font that is wider, reducing the size of the "homescreen" from 21 to 16 rows. Currently, the English-speaking community of Casio calculator enthusiasts does not know how to print Chinese characters from add-ins, even though there is at least one non-official add-in, made by Chinese enthusiasts, which does it. A serious effort to understand how the OS and the add-in in question achieves the effect is yet to be made. It could be that a different set of syscalls would need to be used, but preliminary reverse-engineering attempts have shown that this doesn't appear to be the case.
Seeing the built-in messages
You can see all of the built-in messages and their add-ins, by entering the TestMode (after entering, press 2, 4, then up/down or type a number) or using add-in like this one. Using the first method, each message is shown in the six built-in languages at the same time. Using the second method, only the translation for the currently selected language is shown.
Using the built-in function key labels
Like with built-in messages, the IDs for function key labels have been consistent across OS versions, with new labels being added at the end of the tables, with a ID higher than the previous highest. This doesn't mean existing labels can't change - see the "context" note on the #Using the built-in messages section.
GetFKeyPtr will return the address of the bitmap for a given label ID, taking into account the currently selected language for function key labels. You can find which labels are available and their add-ins using Insight's FKey icons option, or the TestMode (after entering, press 2 then 0, then use left/right up/down or type an ID). On the TestMode, English and Chinese versions are shown side by side. When a Chinese version is the same as the English version, it is shown in green. Possibly, this means that to save space, duplicates are not stored, and a pointer to the English bitmap, or some other indicator, may be used instead.