No doubt ‘international’ users share in your interest and frustration, and fortunately with some care and effort the language translation issue can be overcome…
My thoughts FWIW -
The code is THE direct concrete document to inform those developers not sure how not to run afoul of internationalization best practice.
But since ERPNext is developed for native EN by default (I assume?), of course EN language developers can remain misinformed.
The good news, to me at least? Basic German DE translation test code can be found here!!
https://github.com/frappe/frappe/blob/develop/frappe/tests/test_translation.py
(The code is commented out, my guess since the tests are targeted to run and pass in a DE local environment?)
Additional basic test suites for Turkish TR, Spanish ES, French FR and so on could be implemented by those concerned communities based on the DE test ones here, no?
Another thought - the DE community may want to run (and record) a webinar on that topic, similar this that @Martin_Seibert and team kindly sponsor here Join our virtual documentation sprint - Help us improve the documentation and show use cases to the world!
For reference here are intro docs Docutils Internationalization
The language translation dictionaries can be found like so, for EN and FR:
frappe@ubuntu:~/frappe-bench$ find . -name '*en*.csv'
./apps/erpnext/erpnext/translations/en-US.csv
./apps/erpnext/erpnext/translations/en-GB.csv
./apps/erpnext/erpnext/translations/en.csv
./apps/frappe/frappe/translations/en-US.csv
./apps/frappe/frappe/translations/en-GB.csv
./apps/frappe/frappe/translations/en.csv
frappe@ubuntu:~/frappe-bench$ find . -name '*fr*.csv'
./apps/erpnext/erpnext/translations/fr.csv
./apps/erpnext/erpnext/translations/fr-CA.csv
./apps/frappe/frappe/translations/fr.csv
./apps/frappe/frappe/translations/fr-CA.csv
My last suggestion - an open challenge is to write a test for this notable (stalled?) case you refer to @Tufan_Kaynak2 [v12.1.6] Decimals in Currencies not working