朕惟フニ我カ皇祖皇宗國ヲ肇ムルコト宏遠ニ德ヲ樹ツルコト深厚ナリ我カ臣民克ク忠ニ克ク孝ニ億兆心ヲ一ニシテ世世厥ノ美ヲ濟セルハ此レ我カ國體ノ精華ニシテ敎育ノ淵源亦實ニ此ニ存ス爾臣民父母ニ孝ニ兄弟ニ友ニ夫婦相和シ朋友相信シ恭儉己レヲ持シ博愛衆ニ及ホシ學ヲ修メ業ヲ習ヒ以テ智能ヲ啓發シ德器ヲ成就シ進テ公益ヲ廣メ世務ヲ開キ常ニ國憲ヲ重シ國法ニ遵ヒ一旦緩急アレハ義勇公ニ奉シ以テ天壤無窮ノ皇運ヲ扶翼スヘシ是ノ如キハ獨リ朕カ忠良ノ臣民タルノミナラス又以テ爾祖先ノ遺風ヲ顯彰スルニ足ラン
斯ノ道ハ實ニ我カ皇祖皇宗ノ遺訓ニシテ子孫臣民ノ俱ニ遵守スヘキ所之ヲ古今ニ通シテ謬ラス之ヲ中外ニ施シテ悖ラス朕爾臣民ト俱ニ拳々服膺シテ咸其德ヲ一ニセンコトヲ庶幾フ
明治二十三年十月三十日
御名御璽
And why are the Okurigana written in Katakana? Can average Japanese understand this?
This is the Imperial Rescript on Education (教育に関する勅語) . Go to Wikipedia for an account of its origin and a translation. It's one of the foundational (if that's a word) documents of the Meiji regime. It's not really "standard" Japanese. It's in highly formalised language, a modified form of Classical Japanese. The putative author (although it was composed by advisers) is the Emperor: the first character is ちん, a first person pronoun used only by the Emperor, and the four kanji below the date (30 October 1890) say "Imperial Signature, Imperial Seal". Language like this was used in formal state documents until fairly recently. Look at the Constitution of 1947, which uses a somewhat less demanding form of it, and although now normally printed with okurigana in hiragana was promulgated with katakana okurigana. Most Japanese people would have difficulty with this text if it was suddenly sprung on them, but many will have studied this particular document to some extent at least in history classes. Fifty years ago everybody over the age of about 35 would have been familiar with it, since up to 1945 it was read daily in schools and displayed on classroom walls. You will often find katakana used for okurigana in scholarly writing by authors of a conservative bent right up to the Pacific War period.
