Neither taller nor tallest is correct, as the question is referring to a point. The correct would be highest peak or higher peak. In a higher vs highest (or taller vs tallest) argument, highest (or tallest) would be more correct because the question specifies "of the two". Although this difference is negligible and both would certainly be correct.
Wales isn't a principality since it isn't ruled by a prince. Historically it's more of a debate, but certainly since the United Kingdom has existed, it isn't a principality
in the grand tradition of Jetpunk pedantry, Wales has only one official language, which is Welsh. English has no formal legal protection anywhere in the UK.
The megacity one is incorrect if you are just talking about the city of London, Londons population is less than 10 million. Unless you are including the outer area which doesn't actually count as London, which should be said on the question as it is misleading
Clue: it's in the bit headed 'language'.