From b2evolution manual

This manual page is outdated. NEW PAGE: Create My Own Flag / Locale in b2evolution v5+.

Many users would like to just add a flag for their country, even if they don't use a localized version for their language. Here's how to proceed:

1. Find the [ISO 3166 code] for your country. Let's assume it is XY.
2. Find a [flag for your country] and proportionally resize it to the dimensions b2evo needs. b2evo works with two versions of each flag. One should be exactly 16 pixels wide (height depending on the flag's proportions -- Did you know all flag don't have the same form factor?). The other one should be exactly 10 pixels high (width depending on proportions). The version used in displays depends if we need to line up flags horizontally or vertically.
3. Save the resized flags as xy.gif (remember XY is your contry code), each in it's own folder: /blogs/img/flags/w16px and /blogs/img/flags/h10px.
4. Wait, that's not enough! ;) Flag images are useless until they are referenced by a locale. A locale is a set of regional parameters. here's how to create your own: Find the [ISO 639 code] for the language you're blogging in. If you're blogging in English, you should use en. Let's assume you use en...
5. In the backoffice, go to "Settings", then "Regional" and at the bottom of the page, click on "Create new locale" and enter the following information: (these are default values, you may want to adjust them to your needs):

 Locale: en-XY
 Enabled: checked
 Name: English (XY)
 Charset: iso-8859-1
 Date format: Y-m-d
 Time format: H:i:s
 Lang file: en-XY
 Priority: 1

1. Click "Create" and you're done! :)

Note: The new locale and flag will only work on your own system. If you ping b2evolution.net and appear in the 'recently updated' list there, you should not expect your flag to appear there. However, your locale is transmitted and if it matches an official locale, the appropriate flag will be displayed.

Note 2: Now you've done that part, how about translating b2evo to your language?