The strings used in the UI of Friendica is translated at Transifex  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 message.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 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.
The location of the translated files in the source tree is /view/LNG-CODE/ where LNG-CODE is the language code used, e.g. de for German or fr for French. For the email templates (the *.tpl files) just place them into the directory and you are done. The translated strings come as a "message.po" file from transifex which needs to be translated into the PHP file friendica uses. To do so, place the file in the directory mentioned above and use the "po2php" utility from the util directory of your friendica installation.
Assuming you want to convert the German localization which is placed in view/de/message.po you would do the following.
1. Navigate at the command prompt to the base directory of your friendica installation 2. Execute the po2php script, which will place the translation in the strings.php file that is used by friendica. $> php util/po2php.php view/de/message.po The output of the script will be placed at view/de/strings.php where froemdoca os expecting it, so you can test your translation mmediately. 3. Visit your friendica page to check if it still works in the language you just translated. If not try to find the error, most likely PHP will give you a hint in the log/warnings.about the error. For debugging you can also try to "run" the file with PHP. This should not give any output if the file is ok but might give a hint for searching the bug in the file. $> php view/de/strings.php 4. commit the two files with a meaningful commit message to your git repository, push it to your fork of the friendica repository at github and issue a pull request for that commit.
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.
For further information see the utils/README file.
Friendica uses the language setting of the visitors browser to determain the language for the UI. Most of the time this works, but there are some known quirks.
One is that some browsers, like Safari, do the setting to "de-de" but friendica only has a "de" localisation. A workaround would be to add a symbolic link from $friendica/view/de-de pointing to $friendica/view/de