Originally Posted by Doitsu
Regarding the Primary Dictionary option, I just had a look at the K4 manuals and according to manual there's a Dictionaries option that lets you choose the primary dictionary.
Kindle 4 Touch
Out of curiosity, are the manuals incorrect, are these options only available on registered K4s or do they exist but don't work?
With the KT 5.1 update, K4 and KT should work identically in terms of dictionaries. Each language may have zero or more dictionaries, and when there are two or more for a given language, you can set one as default. Most languages using Latin-based scripts should work, but I've had no luck with, for example, Russian (Cyrillic script). There are some free ones you can try out here (and scripts that you can use to generate your own dictionaries):
Note that the language properties have to be set correctly in both dictionary and in the metadata of text you are reading. If no language property is set, it probably just uses a dictionary corresponding to the UI language. You can set language property for PDF also and Kindle should use that, but I haven't tested it (and K4 and KT use different PDF libraries so there may be differences).
I believe if you have several dictionaries for a given language, they will 'cascade', that is to say, a word not found in default dictionary will be looked up in the second and so on. So it is possible to 'chain' dictionaries with specialized vocabularies, or add foreign word definitions etc. But I need to check this some more. (a French English translation dictionary I'd purchased years ago from Amazon happened to have the wrong lang code defined, and was treated as an English dictionary on KT. I was surprised to find that French words in the middle of English sentences popped up English translations from this dictionary, even though it showed up as a 'secondary' English dictionary. Amazon refunded my money so I could purchase the since-corrected edition of the same dictionary.)
It is possible in HTML to set a lang property for specific blocks of text, but I don't know if Kindle pays attention to that. More experimentation needed...