PA31 PILOT; DISTRACTED BY PASSENGER; DESCENDS 2000 FEET BELOW ASSIGNED ALTITUDE. ATTEMPTING TO CLIMB AND CORRECT THE ERROR CAUSES TCAS RA ABOARD EMB145 AT 15000 FEET.

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MY CLRNC WAS TO MAINTAIN 16000 FT FOR TFC AND EXPECT LOWER ALT AT 30 DME S OF ABC VOR. DURING THE DSCNT; WE MISSED OUR LEVELOFF AT 16000 FT AND STOPPED AT 14000 FT. IN AN ATTEMPT TO RETURN TO OUR ASSIGNED ALT; WE CROSSED FLT PATH WITH OPPOSITE DIRECTION TFC LEVEL AT 15000 FT. HE MADE AN RA CLB FOR AVOIDANCE. WHEN WE SAW THE TFC; THERE WAS NO DIVERSION REQUIRED BY ME. THIS PLANE HAS NO ALT CAPTURE FUNCTION OR ALT ALERTER SYSTEM. ALT HOLD MUST BE MANUALLY ENGAGED. MOST RECENTLY; I HAVE BEEN FLYING A PLANE WITH BOTH OF THESE FUNCTIONS. I FEEL THIS; ALONG WITH THE PAX IN THE COPLT SEAT (WITH NO FLYING EXPERIENCE) ASKING QUESTIONS NEAR OUR ASSIGNED ALT; CREATED THIS ERROR. AS A RESULT; I HAVE CHANGED MY PROCS TO MOVE THE STERILE COCKPIT ALT FROM 10000 FT TO FL180 WHEN I AM FLYING SINGLE PLT. ALSO; I FEEL THAT A POSSIBLE CONTRIBUTING FACTOR IS THE MEXICAN CONTROLLERS TALK TO THEIR MEXICAN NATIONALS IN THEIR NATIVE TONGUE INSTEAD OF THE INTL STANDARD ICAO REQUIRED ENGLISH LANGUAGE WHICH LEAVES ENGLISH SPEAKING PLTS AT A DISADVANTAGE WITH REGARDS TO SITUATIONAL AWARENESS WHEN THEY CONVERSE WITH THEIR NATIONAL CTLRS.