From reCAPTCHA Wiki

There are three ways to internationalize reCAPTCHA:

Built in translations

reCAPTCHA has a number of built in translations. You can use these by setting the lang parameter using the API.

var RecaptchaOptions = {
   lang : 'fr',
};

Custom Translations

If there isn't a translation for your language, you can build your own. You need to translate all of the strings in [1] and set the "custom_translations" variable of RecaptchaOptions. Any strings that you don't set will be taken from the default translation for your language. You can use this to override only part of a default translation (eg, if the translation we chose doesn't fit your region).

Example:

var RecaptchaOptions = {
   custom_translations : { instructions_visual : "This is my text:" }
};

For reCaptcha integrated into a PHP page, pack the line into a script tag and place it before the reCaptha is called.

<script type= "text/javascript">
var RecaptchaOptions = {
custom_translations : { instructions_visual : "This is my text:"}
};
</script>