Translating Jeti is not very difficult, there are some nice tools to do it.
Please add your translation to the translations entry in the [Faq] so other people know what translations there are and there is no double work being done. If your translation is done you can upload it to the sourceforge patch tracker
This page list some java tools for the translation
http://ostermiller.org/attesoro/similar.html
I use the second one on that site:
http://www.solyp.com/binaries/2975/babelfish.zip
Another one is https://resourcebundleeditor.dev.java.net/
Most tools show the untranslated entries with exclamation marks. If your translated entries also have an exclamation mark you have to remove the unfinished translations jeti.properties and jeti_en.properties are always up to date, jeti_de and jeti_nl are also up to date, the other ones probably not.
Please don't use the zaval resource editor, it can't handle the : that are in some keys leading to untranslated entries. further it doesn't warn you when there are untranslated entries, it just fills them in with the default text on save.
The language files are in the jeti/languages directory, just open the jeti.properties file with the editor and add the language you want to translate to and start translating.
There seems to be a problem with the ' before a unicode character. A way around it is to use another character as ' for example u0060 or u00B4 or u2032 (prime)
There is a mailing list for translators, it will notify you of new Jeti versions with new translation entries about a week before their release (Currently there is a new Jeti version per quarter) https://lists.sourceforge.net/lists/listinfo/jeti-translators
If you like GNU Gettext you can use that with java properties by converting the .properties to .mo and then to .po; then you can use a tool to edit the file, then make the .mo file and convert the .mo back to .properties. (man msgfmt and man msgunfmt for more info about it).
If you want to use your translation in the applet you have to add it to the list.txt file in the languages directory. This file should list all available languages (just the two letter code), because the applet can't list the directory