AJAX Register uses language files for its dialogues. To translate it copy the file: language/en-GB/en-GB.com_ajaxregister.ini and rename it to whatever your language code is.
For example, if you are translating it to German you would copy it to: language/de-DE/de-DE.com_ajaxregister.ini. Then translate the strings within the file. Make sure that each string is in one single line and it's best to avoid single quotes ('). To translate the admin part you do the same thing with the file: administrator/language/en-GB/en-GB.com_ajaxregister.ini.
You can also use components like Translations Manager (J1.5) and Localise (J2.5) which can help you translate the files right from your Joomla administration.

To translate the extra fields labels you need to use one word string for the label, like TERMS_OF_SERVICE which you then translate in the language files:
/language/en-GB/en-GB.com_ajaxregister.ini
/language/de-DE/de-DE.com_ajaxregister.ini
etc.

Keep in mind that all files must be saved in UTF-8 encoding.

To upgrade AJAX Register to a new version just install the new package file through your Joomla installer. You don't have to uninstall the old version, just install on top of the old one. The files will be overwritten and all settings will remain.

You use other 3rd party component for registration (like K2, JomSocial, Virtuemart)? AJAX Register will replace Joomla native registration only and won't work with registrations of 3rd party components. You will have to disable registration on the 3rd party component you use. For example, on K2 set parameter "Enable K2 User Profile" to "No" under K2 Advanced Settings parameters.

Yes, AJAX Register works fine with Moovur. Moovur is a Joomla integration of Mollom, an anti-spam service via content analysis and captcha. If using Moovur's captcha, turn off the built-in CAPTCHA and reCAPTCHA.

The Mootools 1.2.4 script added since Joomla 1.5.19 as upgrade plugin doesn't include Tips library. Please replace the file plugins/system/mtupgrade/mootools.js with this file: mootools.js (Mootools 1.2.4 with compatibility layer including Tips library).

AJAX Register is compatible with Joomla 1.5 but the new features like custom fields are not implemented on 1.5. If you use Joomla 1.5 we strongly suggest to migrate to 2.5 or 3.x as 1.5 has reached its end of life and is not supported any more.

Since version 2.3 AJAX Register is no longer compatible with Joomla 1.6 and 1.7 nor any 2.5 version less than 2.5.6. Dropping support for outdated versions of Joomla was necessary in order to achieve compatibility with Joomla 3.x.
If you use Joomla 1.6 or 1.7 we strongly suggest you upgrade to current version of 2.5 because all older versions are unsupported and insecure.
If you must run 1.6 or 1.7 by some reason you can contact us to send you the latest version of AJAX Register that worked with it. But please note that this version will not be supported and it will not have all features of the latest version for Joomla 2.5.

The image captcha is created in "tmp" directory. Make sure nothing is blocking access to this directory, like server settings or .htaccess rules. Some security components like Admin Tools may build rules for disallowing direct access to some directories. In that case add an exception for "tmp" directory under "Allow direct access, except .php files, to these directories".

To validate a text field against a specific rule you can use custom validation in form of regular expression encapsulated in slashes.
For example, to only accept numeric values you would enter something like: /^[0-9]+$/ in the "Regex" field.
If you want your field to be 3 digits like "123", and validated against that, you would enter: /^[0-9]{3}$/
To accept a code beginning with a letter, followed by 6 digits you would use: /^[A-Z][0-9]{6}$/i
More info on regular expressions can be found here.
Please be careful with this feature as entering the wrong regular expression may break the registration functinality. Do not test on the live site.
Also note that compiling a regular expression for your needs does not go under regular support. If you need our help on this it will be charged as custom work.

Make sure you haven't turned off "Asynchronous Check" setting in your AJAX Register options. Also make sure that you haven't specifically disabled Mootools JS library with some kind of Mootools disabler plugin. Finally, check that you don't have JS errors on your registration page which may prevent it from working.

When user is logged in, his extra information (custom field values) is attached to his user object. You can access it like:

$user = JFactory::getUser();
print_r($user->extra);

For example, to access your field with label companyName you will do it with:

$user->extra['companyName'];

If you want to get extra info for different users (other than the logged in one) you can do something like this:

$user = JFactory::getUser($userid);
plgSystemAJAXRegister::attachExtraFields($user);
print_r($user->extra);