How do you pronounce "developmental"?

In American English: /dɪˌvɛləpˈmɛntəl/ (di-vehl-uhp-MEHNT-uhl). In British English: /dɪvˌɛləpmˈɛntəl/ (div-ehl-uhpm-EHNT-uhl). In Canadian English: /dɪˌvɛləpˈmɛntəl/ (di-vehl-uhp-MEHNT-uhl). In Australian English: /dɪvˈɛləpmˈɛntəl/ (div-EHL-uhpm-EHNT-uhl).

Is "developmental" pronounced differently in American and British English?

Yes. In American English it's pronounced /dɪˌvɛləpˈmɛntəl/ (di-vehl-uhp-MEHNT-uhl), while in British English it's /dɪvˌɛləpmˈɛntəl/ (div-ehl-uhpm-EHNT-uhl).