Install built-in system languages
If your question is about global language support, you might want to check How do I add a new language?

If you however simply refer to keyboard layouts: This very much depends on the keyboard/IME used. There is e.g. TouchPal Keyboard available on the playstore -- and also a lot of language packs for e.g. Hebrew, Spanish, Russian, or even handwriting. Same applies to some other keyboard apps, like e.g. AnySoftKeyboard (which even offers a pack for BB-Codes used in forums), Ice Cream Sandwich Keyboard, Perfect Keyboard, and others.

Though your question says you are probably rather looking for the former: If it is just for the keyboard, the latter is the easier approach. While installing a complete language pack may provide you with support for your language in other apps, you should be aware the language must also be explicitly supported by that app itself.