i18n: extend locale/README.md and add translation workflow detail

locale/README.md

@@ -1,12 +1,81 @@
-On my Windows wsl2 (more generic explaination is needed):
+# Pyfa Internationalization(i18n) and Localization(l10n)
 
-`find ./gui -iname "*.py" | xargs pygettext3 -d lang -o locale/lang.pot`
+Below is a summary of [GNU gettext](https://www.gnu.org/software/gettext/) manual, adapted for Pyfa i18n workflow. 
 
-This will generate the pot that should be carried over to the various language directories and renamed .po
+[Poedit](https://poedit.net/) offers a nice GUI for same GNU gettext translation workflow.
 
-`msgfmt -o lang.mo lang`
+## i18n with command line
 
-Run in each language directory, will compile the .po files to .mo files
+Windows users can get these tools via Git for windows, Msys2 or Cygwin; or just use WSL / WSL2.
+For Linux and macOS users these tools might be avaliable out-of-box.
 
-## Issues
-`zh_CH` doesn't seem to work. AddCatalog is not functioning. See https://discuss.wxpython.org/t/localization-not-working-with-zh-ch-addcatalog-returns-false/34628
+### To generate new template for translation:
+
+```console
+$ find */ *.py -name "*.py" | xgettext -o locale/lang.pot -d lang -k_t -k_r -
+```
+
+explaination:
+
+* `find */ *.py -name "*.py"`: collect all `.py` file path in root dir and all sub-folder, write it to stdout
+* `xgettext`: a utility looking for keyword and put string literals in a specific format for human translation
+    * `-o locale/lang.pot`: let `xgettext` write to `locale/lang.pot`
+    * `-d lang`: default language domain is `lang`
+    * `-k_t -k_r`: besides default keyword (including `_`, see `info xgettext` for detail), also look for `_t` and `_r`
+    * `-`: let `xgettext` to read from stdin, which is connected to `find` stdout
+
+this `locale/lang.pot` is called PO template, which is throwed away once actual `ll_CC/LC_MESSAGES/lang.po` is ready for use.
+
+### To initalize PO file for new language
+
+```console
+$ msginit -i locale/lang.pot -l ll_CC -o locale/ll_CC/LC_MESSAGES/lang.po
+```
+
+explaination:
+
+* `-i locale/lang.pot`: input file location
+* `-l ll_CC`: target locale. `ll` should be a language code, and `CC` should be a country code
+* `-o locale/ll_CC/LC_MESSAGES/lang.po`: output file
+    * `ll_CC`: same as above
+    * `LC_MESSAGES`: GNU gettext conventional path to search for localized messages
+    * `lang.po`: language domain and file format
+
+this `locale/ll_CC/LC_MESSAGES/lang.po` should be checked into VCS, later it will be converted into mechine readable format (MO).
+
+### To update PO file for existing translation
+
+```console
+$ msgmerge locale/ll_CC/LC_MESSAGES/lang.po locale/lang.pot
+```
+
+### To do actual translation
+
+just edit the `lang.po` file :)
+
+### To generate mechine readable MO file
+
+For a single locale:
+
+```console
+$ msgfmt locale/ll_CC/LC_MESSAGES/lang.po -o locale/ll_CC/LC_MESSAGES/lang.mo
+```
+
+For all avaliable locale:
+```bash
+for $f in locale/*/; do 
+    msgfmt $f/LC_MESSAGES/lang.po -o $f/LC_MESSAGES/lang.mo
+done
+```
+
+## i18n with Poedit
+
+### To update PO file for existing translation
+
+1. open a existing `locale/ll_CC/LC_MESSAGES/lang.po`
+2. *Catalog* -> *Update form POT file*
+3. select pre-prepared `lang.pot` file
+
+### To translate and generate MO file
+
+edit the translation and hit Save :)