i18n is a programming abbreviation of Internationalisation. The i is the first letter of the word internationalisation, the n is the last letter of the word internationalisation, and the 18 is the 18 letters of the word internationalisation between the i and the n. Clever huh, but if like many a newcomer you don't know that I bet you spent a lot of time trying to find information about translation? Hopefully this page will put you right. Searching for i18n on any of the Tiki Project Sites will reveal there really is quite a lot of help on the subject.
Bidirectional, often abbreviated to bidi, is the term used for sites that accomodate both Left to Right LTR and Right to Left RTL languages. As LTR languages and language users are in the majority the task of making sites bidirectional is often assumed to be accomodating RTL and RTL is therfore used as another name for the task.
Internationalisation , often abbreviated to i18n, is the term used in the various wiki and other software development communities for taking web offerings and software away from a host or single language and out into the wider world. This might not be the first word that came to your mind when looking for information about translation, but it makes sense if you think about it. Translation might not be the only way of making a web thing available to people in other countries.
LTR stands for Left to Right and is used as the antonym of RTL, Right to Left, in the context of which direction a script runs on a page or other medium.
RTL stands fo Right to Left and is used as the antonym of LTR, Left to Right, in the context of which direction a script runs on a page or other medium.