Finding out which characters are supported
There is an increasing number of Android devices now available and it is sometimes difficult to assess if a particular language is supported and whether all the desired characters are available such as, for example, characters from the IPA (International Phonetic Alphabet - see the IPA Symbol Equivalents for a detailed list).
One of the key elements needed for proper support of a particular language is the availability of fonts covering the corresponding shapes and behaviours. Since mobile devices, such as phones or tablets, are not intended for the same usage as laptops or desktops, they tend to work from different assumptions when it comes to writing systems support, both in terms of display and input methods. Thankfully a number of options exist to help check if a particular Android device supports the desired characters and can be used in a particular language.
Applications to check existing support and coverage
Unicode Map can be used to browse through the many Unicode blocks and search for the desired characters. Characters which are available will be rendered using the installed fonts, while characters which may already exist in the Unicode standard, but for which no available font provides coverage on the device, will simply appear blank. It will then allow copy-and-paste into other applications, such as a text editor or a browser, for example.
These applications will facilitate searching and isolating specific Unicode characters to find out whether or not they are supported at the system-level.
Web-apps making using of web fonts via @font-face
Alternatively you can rely on using a web application with support for web fonts via the @font-face standard to bypass the existing font limitations. The needed fonts will be downloaded on-demand by the browser as assets to the web page.
Such web apps using web fonts need recent browsers such as Firefox or Chrome. Refer to the web app in question for the precise details of the supported fonts; where support is available the fonts are unlikely to work outside of the browser.
Installing new fonts as separate packages or add-ons
Certain fonts are installable as chargeable add-ons directly from the Google Play Store, but this is highly dependent on how the manufacturer has chosen to set up the device.
Installing new fonts system-wide
The possibility of adding your own fonts to be used throughout the whole system is still very limited: it's a complicated process involving replacing the existing fallback font system for which write access to the system folders (namely /system/fonts) is required. This is only available to rooted/jailbroken devices. Such a procedure is recommended only for advanced users.
Bundling and shipping fonts within an application for developers
Developers - or anyone involved in the creation or commissioning of applications - can bundle and ship fonts directly into an application as part of its assets, to provide on-the-spot support for a particular script. Android allows such bundling within applications (given proper permissions from the font creator, of course) but the limitations of the platform imply that such a bundled font will only be available to the application itself and not to any other application on the system.
Example of support for IPA (International Phonetic Alphabet)
Recent versions of Android, such as version 4.0 and 4.1/4.2, provide better support for IPA thanks to the ongoing improvements added to the current version of the font families installed by default: Droid Sans and Roboto. For earlier versions of Android which may not be upgradable to the current one, the web apps with web font, or the separate font package add-ons options, should provide fall-back possibilities.