Google Voice Search is dependent on the attribute” lang”. If you set the «en-EN» it will be recognized only in English. “Lang” attribute is set as follows: <Meta http-equiv = 'Content-Language' content = 'en-EN' />.
Surely, PHP coding should be trusted only to professionals. If you have no experience we advise you to ask our assistance since our team works with these algorithms and codes on a daily basis. Most of our premium WordPress themes have the pre-installed Voice Search system developed by Google.