These days I tried to review existing simplified Chinese translations via the translation portal, also made some minor quick fix in my own instance via custom translation, but both approach for translation is far from easy and convenient, because there is only source text available for sorting, no module and doctype based filtering possible, especially when using the custom translation, it is needed to key in the source text!
for a more efficient custom translation, I would suggest the following
make the translation relevant elements(control): field label, column header, status text, form/list/report title etc translatable, by double click on the label or any other attached icon to jump to the new translation form view with the existing source and translated text auto filled, ready to be corrected!
or add module field into custom translation doctype, upon select the module, auto fetch all relevant source and translated text.
@Pawan, @Bas_de_Reus, Yes I do already used the translation portal to verify and edit the existing translation a lot, due to the fact that no sorting and filtering by module or doctype possible, also no batch updating via upload Excel file possible, in the portal, there is no context for the source to be translated, all in all it is not easy to translate using the translate portal!
for real users, they may only use certain features/ module, doctype or screen, if it is possible to directly translate relevant field/caption/title/header, then it is very handy for service providers to serve their customers when target language is not yet complete!