The operating system comes with a Tamil unicode font Inaimathi and supports two keyboard options, namely Anjal (almost phonetic) and Tamil99.
To activate the support for Tamil Language,follow the following procedures.

Step 1. Open System Preferences

Step2. Click the International Option in personal section

Step 3 Click on edit List under Languages Menu

Step 4: Tick the desired language and click ok to go back to International menu

Step 5

Click on Input Menu and choose the Anjal and Tamil99 input Method

Step 6. Close sytem Preference menu.

To use the Tamil keyboard and font in textedit first open the text edit application and then click on the american flag icon on the upper right corner in the menu bar and choose Anjal from the drop down Menu. (optionally the keyboard also can be chosen to get a virtual keyboard.

The American flag will turn into

Now start typing the following key sequence

agara muthala ezuthellAm aathi pagavan muthtrRE ulagu

this will appear on the textedit
அகர முதல எழுத்தெல்லாம் ஆதி பகவன் முதற்றே உலகு (the font is Inaimathi a unicode font).

To switch back to english click at the same anjal symbol (on top right corner) and select US. Thus by clicking on the US symbol one can type in various languages in the same document

Devanagari-qwerty can be chosen in the same manner to type in sanskrit. The Devanagari keyboard layout is as under (pdf here):