I wonder why only I can read my messy letters in spitewhen it'’s very hard to read for other people.

My way of pen gripping the pen might be a little strange.

I think the appearance of Japanese language is cool.

Do you mean written Japanese looks cool? A more natural way to say that may be “I think Japanese just looks cool.” or “I think Japanese as a language looks cool.”

I wonder why only I can read my messy letters in spite it's very hard to read'm the only person who can read my messy handwriting. It's much harder for other people.

My way ofI think my pen grip might be a little strange.

The opportunity to write has reduced these daysIt's getting more and more uncommon to write because of computers. becoming popular.

I think the appearance of Japanese language ilooks cool.

I wonder why only I can read my messy letters in despite it's being very hard to read for other people (to read?).

I wonder why only II am the only one who can read my messy letters in spite ihandwriting. It's very hard to read for other people.

My way of pen gripping the pen might be a little strange.

These days, I have fewer opportunityies to write has reduced these days because of computers.

I think the appearance of Japanese languagescript is cool.

Language means the words/the language in general, script more specifically refers to the written language

I wonder why only I can read my messy letters in despite it'sthem being very hard to read for other people.

Mind the grammatical number. "Letters" is plural, so you should use "them".

MyThe way of pen gripI grip the pen might be a little strange.

You can also say "my pen grip", but I personally prefer "the way I grip the pen".

The number of opportunityies to write has reduced these days because of computers.

(1) Opportunities can't be "reduced", but the NUMBER of opportunities can.
(2) You're talking about opportunities in general, rather than one specific opportunity, so the plural form ("opportunities") is expected.

I think the appearance of the Japanese language is cool.

We will either say "the Japanese language" (with the "the") or just "Japanese", nothing in between.

I wonder why onlySMOOTHER: Although I cmanage to read my messy letters in spitescrawl, it's very hard to read for other peoples to read.

My way of pen grip might be a little strange/unusual.

The opportunity to write has reduced these days because of computersSMOOTHER: With the advent of computers, fewer and fewer people write by hand.

I think the appearance of Japanese language is coolOPTION A (with British spelling): However, I am fond of the aesthetics of Japanese scrips.¶
OPTION B (with American spelling): However, I admire the esthetics of Japanese writing.