From MozillaZine Knowledge Base
This article describes how to view and type in languages other than your system's default language.
Viewing international characters
To view international characters, you do not normally need to do anything special in Thunderbird. Your operating system must have fonts for the languages that you want to view. You might have to download and install fonts in your operating system. Some downloads include tools for typing other languages, but to view other languages you only need the fonts.
Some e-mails are badly formed. This can happen if the program that sends the e-mail has a bug or is badly written.
If you see the wrong characters in an e-mail, then you might be able to fix it by choosing View -> Character Encoding. You will have to find the right character encoding by trial and error.
If you receive many e-mails that are badly formed in this way, you can use a folder to fix the character encoding. Make a folder and choose Edit -> Folder Properties. Set the default character encoding for the folder and check the box "Apply default...". Now you can fix messages by moving them into this folder.
To print messages that are badly formed in this way, you must use the folder method.
Note: If you have problems with character encodings, choose Tools -> Options -> Display -> Formatting -> Fonts. Ensure that the checkboxes in the Character Encodings section there are not checked. (If they are checked, then they force Thunderbird to use the wrong character encoding for some messages.)
Typing international characters
To type international characters, you can use your operating system to change your keyboard mapping for the language that you want to type. For languages that do not easily map to your keyboard, (for example, Japanese on an English keyboard) you can use an input method editor (IME). If your operating system does not provide these things, you can usually download and install them.
If you are using Windows and English you can use the US-International keyboard layout to enter both English and accented characters. This doesn't require any different hardware, just changing what software keyboard layout you use in Windows. That lets you enter US characters normally, and international characters by using a strike-over feature. If you enter a apostrophe ('), quotation mark ("), accent grave (`), tilde (~), accent circumflex, or caret (^) key nothing happens until you press a second key. If the second key is a letter that supports an accent mark, it types the appropriate accented character. If you pressed a ineligible key it displays the two keys you typed instead. The advantage of this method is its quicker/easier to enter any accented letters that it supports than trying to figure out a ALT code, Unicode value or look up a character in a table. See https://support.microsoft.com/en-us/help/97738/using-us-int-l-keyboard-layout-to-type-accented-characters for more information.
You could also use the Windows character map (it's in "Accessories -> System Tools" if you use Windows XP) , Thunderbird's "Insert -> Characters and Symbols" menu command, or use the ALT key plus the character code on the numeric keypad if you're running Windows. There is a table listing the symbols for ALT codes here. For example, if you hold the ALT key and type 225, ÃŸ (Esszet or sharp S) will be displayed when you release the key. https://support.microsoft.com/en-us/help/315684/how-to-use-special-characters-in-windows-documents describes how to type a character using its Unicode value.
According to https://superuser.com/questions/1142924/umlaut-Ã¤-Ã¶-Ã¼-on-english-us-layout both Linux and OS X support a US-International keyboard layout, letting you use the same mechanism to enter international characters. It doesn't work identically though, some of the key combinations are different. https://superuser.com/questions/154303/is-there-a-us-international-keyboard-layout-on-linux-that-mimics-windows-behavi describes how to "configure the effect of dead keys by creating a file called .XCompose (note CApitalization) in your home directory and listing the combinations you want".
Linux supports a character map application that you can copy/paste characters from. A quicker solution would be to type a character by typing it's Unicode value in hexadecimal using Control+Shift+U or enter a Compose key sequence (requires the Gnome Tweak Tool). See https://www.maketecheasier.com/quickly-type-special-characters-linux/ .
OS X supports a Emoji & Symbols Viewer that you can copy/paste characters from. It also supports a Character Picker application and the ability to use the Option key to enter special characters. See https://www.maketecheasier.com/quickly-type-special-characters-linux/
If you want to use a right-to-left language such as Hebrew or Arabic install the BiDi extension.
The Quick Locale Switcher lets you quickly switch what language is used in both the user interface and by the spell checker. You have to restart Thunderbird if you want to switch the language used in the user interface but don't have to do that just to change the dictionary.
The Simple Locale Switcher add-on also requires you to restart after changing locales.
The Automatic Dictionary Switching add-on remembers the language of the spellchecker used when composing and switches to it every time you write to that person
The Dictionary Switcher add-on automatically switches the dictionary based on the language used in the message if you set extensions.dictionary-switcher.autodetect true using the config editor. That web page is in the Firefox section but the same add-on works for Firefox, SeaMonkey and Thunderbird.
Thunderbird's user interface
The Mozilla localization project page has links to localization efforts (other languages) that are not listed in the Thunderbird download page . BabelZilla is a independent user community of developers and translators of extensions for Mozilla applications. It has a web translation system "to allow online translation and manage upload/download of extensions & languages files."
International characters are supported in domain names (IDN) but they are not supported in the username portion of a email address. Non-ASCII characters in the username isn't part of the IDN spec, its part of supporting UTF8 in headers in RFC 6532 (Internationalized Email Headers). Thunderbird doesn't support RFC 6532 yet.