This document provides guidelines for internationalization of the Bitcoin Core software.
To mark a message as translatable
In GUI source code (under src/qt): use tr("...")
In non-GUI source code (under src): use _("...")
No internationalization is used for e.g. developer scripts outside src.
On a high level, these strings are to be translated:
Do not translate technical or extremely rare errors. Anything else that appears to the user in the GUI is to be translated. This includes labels, menu items, button texts, tooltips and window titles. This includes messages passed to the GUI through the UI interface through InitMessage, ThreadSafeMessageBox or ShowProgress.
Try not to burden translators with translating messages that are e.g. slight variations of other messages. In the GUI, avoid the use of text where an icon or symbol will do. Make sure that placeholder texts in forms do not end up in the list of strings to be translated (use <string notr="true">).
Try to write translation strings in an understandable way, for both the user and the translator. Avoid overly technical or detailed messages.
Do not translate internal errors, log messages, or messages that appear on the RPC interface. If an error is to be shown to the user, use a translatable generic message, then log the detailed message to the log. E.g., "A fatal internal error occurred, see debug.log for details". This helps troubleshooting; if the error is the same for everyone, the likelihood is increased that it can be found using a search engine.
Avoid dividing up a message into fragments. Translators see every string separately, so they may misunderstand the context if the messages are not self-contained.
There have been difficulties with the use of HTML in translation strings; translators should not be able to accidentally affect the formatting of messages. This may sometimes be at conflict with the recommendation in the previous section.
Plurals can be complex in some languages. A quote from the gettext documentation:
In Polish we use e.g. plik (file) this way:
1 plik,
2,3,4 pliki,
5-21 pliko'w,
22-24 pliki,
25-31 pliko'w
and so on
In Qt code, use tr's third argument for optional plurality. For example:
tr("%n hour(s)","",secs/HOUR_IN_SECONDS);
tr("%n day(s)","",secs/DAY_IN_SECONDS);
tr("%n week(s)","",secs/WEEK_IN_SECONDS);
This adds <numerusform>s to the respective .ts file, which can be translated separately depending on the language. In English, this is simply:
<message numerus="yes">
<source>%n active connection(s) to Bitcoin network</source>
<translation>
<numerusform>%n active connection to Bitcoin network</numerusform>
<numerusform>%n active connections to Bitcoin network</numerusform>
</translation>
</message>
Where possible, try to avoid embedding numbers into the flow of the string at all. E.g.,
WARNING: check your network connection, %d blocks received in the last %d hours (%d expected)
versus
WARNING: check your network connection, less blocks (%d) were received in the last %n hours than expected (%d).
The second example reduces the number of pluralized words that translators have to handle from three to one, at no cost to comprehensibility of the sentence.
During a string freeze (often before a major release), no translation strings are to be added, modified or removed.
This can be checked by executing make translate in the src directory, then verifying that bitcoin_en.ts remains unchanged.