Is true that metal halide is a mercury vapor lamp with halide salts added?

True. The first MH lamps, had plain MV arctubes (With the starting resistor), but shorter, and had mercury and metal halide salts to improve the color rendering and lumen efficacy. Modern MH lamps, have different arctube designs without the starting resistor, and requires an ignitor similar to the high pressure sodium lamps. Today, there are also metal halide lamps with ceramic arctubes, that allow increasing the temperature of the metal halide salts and improving color rendering and lumen efficacy.
The american developed metal halide lamps with mercury, sodium and scandium iodides. The european favorited the tri-salt with mercury and indium, thallium and sodium iodides. Osram favorited with mercury and dysprosium and thallium.

In addition :

The Ceramic arctube allows the use of Halides which would damage the original Quartz arc tube, making wider choice available

The types of Halides used are no longer unique to the US or Europe. Many of the more advanced lamp types are available for all countries

The MH really started as an gradual improvement, so evolution of MV. But then the "MH-special" problems were discovered and that caused the MH's to depart from the MV technology (so needing separate ballast, using saturated vapor instead of fill dosage pressure control, getting rid of the starting probe so becoming pulse start only,...), till they became more closer relatives to HPS (ceramic arctube, saturated vapor pressure control, strictly pulse start,...; most modern types are then even electrically compatible with European HPS).