I encountered the following problem: I was building a multi-lingual website with three languages, powered by the popular WPML plugin (affiliate link). I had the regular language switcher set up in a menu. Now, when I was looking at pages in the main language – in this case English – the two other languages, Romanian and Russian, were elegible in the language switcher. Whenever though I switched the language to Romanian, the Russian selector went missing. This wasn’t the case the other way around, though: Looking at Russian pages, all three languages showed up in the switcher.
The fix was quite easy: I only had to clear the WPML cache. In order to do this I had to navigate to WPML > Support > Troubleshooting in the WordPress dashboard. Here, under the heading “Clean up” was the option to “Clear the cache in WPML”. This was all it took – just remember to backup your database beforehand.