I can read Japanese and English, but I don't post in Japanese forums because I use Japanese all day every day and I am tired of it, to tell you the truth.
I try to brush up on my English skills, so I write in English forums.

And to tell you the truth, it takes a lot of time to read both forums.

A quick summary of what I have seen:
People on JP servers also want GC removed from Frontlines.
Some people want the Contents Finder (CF. In English Duty Finder) to be separated by "already cleared" and "not cleared yet".
A thread about how it's difficult to get people to do Bahamut raid
There is one thread (508 pages now) just for listing things you want added to the game.
There is a thread about adding a "password" feature to Party Finder listings, so you can only join a party if you know the password. I guess this is to let people from other servers join your party if Party Finder becomes cross-server.
A thread about changing the effect of Astrologians Benefic. (I don't play AST, so I don't really understand what that skill does.)
There is a thread to debate problems with Dark Knights and propose changes to that class.
I've seen at least 2 threads asking for more severe penalties on people leaving a dungeon halfway through.

Just like in the English forums, many people feel differently about a lot of things.

There are some major differences I've noticed.
In JP forums, people tend to continue to write in one thread for along time. Even over years. It doesn't seem like people create 3, 4, or 5 threads about the same topic.

People don't seem to be complaining about quality of players or problems with groups in Contents Finder. (Unlike the long thread about Tales of the Duty Finder on the ENglish forums.) This is because, in general, Japanese put the needs of the group over the needs of the individual. In Japanese society, we think "Is what I want to do going to inconvenience other people". If so, we do not do that. (It is important not to cause trouble for others. It's basic manners and cornerstone of living on crowded a small island nation. Of course people want harmony. But more importantly, we need harmony and cooperation to live together)
But of course, there are some people who do not care. Like the ones who leave parties halfway through.

No threads about improving the community or complaining about hunts, that I could find at least. However, there are many, many threads like this on English forums.
Haven't seen any threads complaining about mentors.

But, like I said, some threads are 300 - 500 pages long and are years old I haven't read everything, and I won't. I don't know everything what is talked about in JP forums.