[LarzI]

Under the command assign a bool array for the player to true

pawn Код:

g_bMyBool[ targetid ] = true;

Then check if that bool is set under OnPlayerText, and loop through the string and use https://sampwiki.blast.hk/wiki/Tolower to set each character to lowercase:

pawn Код:

if( g_bMyBool[ playerid ] )
{
    for( new i = 0, j = strlen( text ); i != j; i ++ ) //128 is maximum chat output
    {
        Tolower( text[ i ] );
    }
}

You obviously need to declare and name that bool array all by yourself, but I'm sure you can handle it, as I don't believe you don't have any knowledge at all.