Aha, it seems to be quite clear from the screenshot now . You have selected English language for your interface and editing English master language package through the web has always been prohibited. If you want to translate Moodle into a non-English language, change the UI language (at the Moodle home page, for example) to that language. If you want to modify official English translation, switch the lang.php into the "_local" mode (there is a button for changing the mode in 1.8.3). See Translation docs page for more info about *_local packs.
If you mean translating seriously, I would recommend to run a 1.9 installation as the lang tool has been improved in several ways (e.g. in translating additional plugins).
Hope this helps.
p.s. the string langnoeditenglish should be displayed as "'Can't overwrite the base English language pack. Please switch lang directory to make local changes". There is something strange in your installation