Edit and create new instruments

If you right click a staff, select Staff Properties, and you can change the name and transposition. To change the sound used for playback, use the Mixer window. the soundfont that comes with MuseScore has only the standard General MIDI set of instruments, but if you have a different soundfont that includes samples of the instruments you want, you can install that instead - just see the section on soundfonts in the Handbook. You can also save any score created this way into the templates folder to use as a basis for future scores. You can also create a customed version of the instruments.xml file in that folder to make those instruments available in the instrument list.

The definition of the instruments is stored in instuments.xml resp. the file selected in menu->Edit->Prefecences...->General->Intruments list and can get changed there.
A comfortable way to edit this is in being talked about for the next version of MuseScore, see http://musescore.org/en/developers-handbook/references/musescore-2.0-ro…

The instruments are defined in instruments.xml file. You can change which file MuseScore will read on startup in Edit -> Preferences -> General -> Instrument Lists. Click the blue folder and locate instruments_es.xml. Restart MuseScore and try to create a new score with the new score wizard. You should see instrument names in spanish, maybe the galician bagpipes will be there.

If they are not in, go to the MuseScore installation folder, in the templates directory. The full path should be something like C:\Program Files\MuseScore\templates. Open the file instruments_es.xml with a text editor such Notepad++ or PsPad (don't use Notepad or Wordpad).

This file is an XML file, it contains each instrument category and in each category the list of the instruments. At the bottom, there is
{syntaxhighlighter brush:xml; gutter: false;}

{/syntaxhighlighter}

You could copy paste this for galicians instruments or add more instruments in the woodwind or brass instrument groups. Note that the extended flag if present and set to 1 will make the instrument group invisible except if you click on "Show more" in the new score wizard.

To create a new instrument add the following in one of the instrument-group
{syntaxhighlighter brush:xml; gutter: false;}

Piccolo
Picc.
1
74-105
74-108

72

1

{/syntaxhighlighter}

Name and short name are self explanatory. They are used in front of the staff and it's the only thing that should be in spanish or gallego.

Clef defines the default clef for the instrument.
0 : G clef
1 : G clef with 8 alta
2 : G clef with 15 alta
3 : G clef with 8 bassa
etc...

aPitchRange and pPitchRange are the amator and professional range of the instrument in midi pitches

program is the General MIDI program number that will be used to selec the sound in the soundfont. It's 0 indexed : Acoustic Grand Piano. See General MIDI for the other ones.

extended. Omit it if you want the instrument to appear without Show More selected.

This description is not complete. Unpitched percussions are slightly different. Some instruments like violin and trumpet can define several sounds and so several channel and programs. And you can also define some articulations per instrument.

And this post should go into an how to :)