The strings used in the UI of Friendica is translated at [Transifex] [1] and then included in the git repository at github.
If you want to help with translation for any language, be it correcting terms or translating friendica to a currently not supported language, please register an account at transifex.com and contact the friendica translation team there.
Translating friendica is simple.
Just use the online tool at transifex.
If you don't want to deal with git & co. that is fine, we check the status of the translations regularly and import them into the source tree at github so that others can use them.
We do not include every translation from transifex in the source tree to avoid a scattered and disturbed overall experience.
As an uneducated guess we have a lower limit of 50% translated strings before we include the language (for the core messages.po file, addont translation will be included once all strings of an addon are translated.
This limit is judging only by the amount of translated strings under the assumption that the most prominent strings for the UI will be translated first by a translation team.
If you feel your translation useable before this limit, please contact us and we will probably include your teams work in the source tree.
If you want to help translating, please concentrate on the core messages.po file first.
We will only include translations with a sufficient translated messages.po file.
Translations of addons will only be included, when the core file is included as well.
If you want to get your work into the source tree yourself, feel free to do so and contact us with and question that arises.
The process is simple and friendica ships with all the tools necessary.
Additional to the po2php script there are some more utilities for translation in the "util" directory of the friendica source tree.
If you only want to translate friendica into another language you wont need any of these tools most likely but it gives you an idea how the translation process of friendica works.