When I left WordPress development years ago it was because of messy code, plugin dependency hell, update nightmares and lack of scalability - not to mention vulnerability. Even after years I spent more time with bugfixing and finding workarounds rather than actual development. In this particular case the rewrite was not an option but a must, even though the site I inherited was well written and organized. A critical plugin was generating so large database traffic that made the hosting provider issue a shutdown warning.
This is where we decided a complete rewrite in ProcessWire, that has multi-language support out of the box (and beeing one of the bests I have seen so far). Of course no CMS is perfect but I'm confident that it will cope with the challenges better. So far so good: the site is snappy, editors can manage multilanguage content easily and it's all responsive. From a developer point of view it was a joy to work with even if certain features needed to be added manually rather than just installing another plugin. Plus there's no shutdown notice message yet :)