Yosemite changes QWERTY keyboard layout I have a qwerty keyboard, but I type in french and japanese quite often.

Up until now, I used to have kotoeri activated with Romaji writing for english and french. I used to write accentuated letters by :

option + e + letter for  é, ó, etc… 
option + ` + letter for à è ù etc…
option + i + letter for î, û, ê, etc… 
option + u + letter for ü, ï, ö, etc… 

Under yosemite, this doesn't work anymore.

To get it back I had to activate US International – PC. But it still doesn't work properly. There is a huge bug :

when typing ' then a letter, I get á, é, etc… This is preventing me for writing properly stuffs like

l'arrosoir  → lárrosoir
qu'un → quún

There are a lot of stuffs like this in french. And even worse, auto-correction kicks in and transforms:

quún in quant

How can I fix this ? How can I still continue typing without learning a new keyboard layout ?

CLARIFICATION
=============

I am talking about several separate bugs.

Bug on U.S. International PC keyboard layout

While using this keyboard pressing option + e then a vowel produces the right output (for instance á) the key 'actually adds an accent to a letter instead of producing the 'character (for instance quelquúninstead of quelqu'un). This is most inconvenient as the character ' alone is quite frequent in french.

Bug on U.S. keyboard layout

Hitting option + e then a vowel has no effect (same for option + u then a vowel, etc…)
→ Solved by deactivating and and activating again this configuration. This is the only working configuration.

Bug on kotoeri

Kotoeri configuration let the user choose which keyboard configuration to use for romaji. I have selected U.S. as usual, and I have the same issues as with the U.S. configuration below. If I select US International PC, nothing changes, I don't even get the behavior of International PC without kotoeri. This is most definitely wrong.

UPDATE
======

Selecting US Extended yields the desired behavior (the old one from US international PC). However, US Extended is not available for kotoeri.
As far as I know, Yosemite Romaji IM's failure to honor option and shift dead keys is a bug that Apple will have to fix via an OS update.  It has been reported elsewhere (e.g. https://discussions.apple.com/message/26864874#26864874  ).  It would be good to use Apple's feedback channel to encourage quick action on this.

The OS X 10.10.1 update does not fix this bug in Kotoeri, but the 10.10.2 update of Jan 27, 2015 DOES fix it.

The behavior described for the US International PC keyboard layout is not a bug, but intended.  People who use this keyboard want the ` ~ ^ " ' keys to be dead keys for producing accented characters, and they learn to add a space when they need the stand alone version.

I have not seen any other reports of the US keyboard layout not working right in Yosemite.  When the US flag is showing at the top right of the screen, you should be able to use the standard option dead keys to produce accented characters without problems.  You should also be able to use the Character Picker, where you hold down the key for the base letter and a popup menu appears where you can select accented versions.  If that is not the case for you, the OS is broken somewhere and may need to be reinstalled.