I'm currently working on 2 WP sites. One is the sub-site of a world-wide IT organization, the other one is the main site of a local business. I had forgotten how site localization can be a pain, so here is a summary of the solutions I'm using for the 2 sites.
The main issue with any kind of site (be it WP or any other system) is access rights. If you don't have full access rights (or only limited access), you can only do what the client allows you to do. Depending of the l10n savviness of the client, that can be a lot or very little.
So let's say that you work directly for the client and you have full access rights to the site.
First you don't want to work directly on the site. You want to install the site on your machine and have it run locally to be able to see all your changes and validate them before you reproduce them online.
Also, you want to be able to manage your changes and know exactly what was that change your made for. You want to have granular control of all the modifications and for that you'll need a version control system. That way you can revert any modification any time you want.
- Follow the instructions here to install a "blank" WP site locally and to be able to run it: https://codex.wordpress.org/Installing_WordPress_Locally_on_Your_Mac_With_MAMP
- (Install and) Use the "All-in-One WP Migration" plugin on the online site to export all the data (an FTP download only won't work, there are databases, etc. that you can't access from FTP)
- Use that same plugin on the local site to import the site
- Now you have a copy of your online site running locally. Changes that you'll do locally will be live right away so that you can test them. They will not affect the online site.
- To have granular control of your modifications, use git to create a version control repository for your site: enter the root of your site and run "git init", add all the files to the repository.
A few links to get up to speed with git:
- You need a robust backup system. Something that does everything automatically so that you don't loose too much data (or any data at all) in case of problem. Time Machine is good but don't hesitate to use something more sophisticated.
- You can now start working on the site localization.
WP is a mess when it comes to translating a whole site. There are no integrated mechanisms to access all the localizable parts: you have to use plugins to create the l10n infrastructure, then plugins to export the theme strings, then whateverelse to actually access the textual contents of the site.
I'll write more later about the second phase of the l10n: the actual translation process.