The first time Nebulb is executed on a computer, the program language is automatically selected according to the language in which the operating system is configured.
Thus, if the operating system is in Spanish (in any of its variants) Nebulb is displayed in Spanish. And in the case of being in any other language (other than Spanish) Nebulb is shown in English.
Apart from this, it is possible to manually select the language of Nebulb at any time (through the menu "Edit -> Language").