An enterprising person [over here][1] has provided a translation table for the letters and some of the numbers, in multiple typefaces:

[![Konosuba translation table][2]][2]

Someone somewhere may have put together a more complete version; this version is from just after episode 3 or 4, and more sourcetext became available as the show went on. 

As you surmise, there is a simple one-to-one relationship between the English alphabet (and related glyphs like punctuation and numbers) and the _KonoSuba_ alphabet. However, beware: the actual text that is enciphered is not necessarily in the English language - some of it is also in romanized Japanese. For example, someone in the linked thread deciphered this part of your image:

[![EXPLOSION][3]][3]

As this:

    koumazokunonakademouiitsunotensaimahoutsukai. b
    akurestumahoutoyobarerusaikyomahounomiruyokunimor
    itsukare. soreshikatsukawanaishitukawanai

Which may seem like gibberish, but in (somewhat typo-laden) Japanese, it's Megumin babbling about EXPLOSION. Roughly, "A wunderkind magician, even moreso than the average Crimson Demon. She is enraptured by the mightiest of magicks: EXPLOSION. She won't use any other magic, and plus - she _can't_." This particular text is cobbled together from various bits of copy about her character, as far as I know. 

  [1]: http://www.matolabel.net/archives/53312283.html
  [2]: https://i.sstatic.net/jZQ65.jpg
  [3]: https://i.sstatic.net/5WcmS.jpg