Has anyone else noticed, that since they started learning Welsh, their spelling in their native language has become worse?
I found myself trying to spell ‘does’ in english the other day and everytime i spelt it i thought, ‘thats does as in Does dim, not the English does’. I think I deleted it and retyped the sentence about 4 times before I realised I was giving it the Welsh pronunciation.
You’re being assimilated… 
Not with writing but with reading, yes, more and more often. I have a fortune my native language isn’t too similar with neither English nor Cymraeg but when I’m speaking my tongue I started to mix Cymraeg words into normally Slovene sentence and have hard time to think about well, wait what’s the Slovene word for this???
Ummm … this could very easily go into You know you are learning Welsh when topic though. 
Sorry, I’m never very good at working out where things go on forums.
Don’t worry. This was just a thought since there you can read quite odd things happening to people who are learning Cymraeg, including me. 
YES!! I was trying to explain this to my family last week, after I had written something in English surprisingly riddled with spelling errors! 
If I’ve been teaching Welsh all day and then go online and write something up in English, there will be some very strange spellings, especially, as you said, ‘does’. The most bizarre one is spelling ‘I’, as in ‘I am Mererid’ as ‘Ai am Mererid’. I was fine before I started tutoring Welsh!
You’re being assimilated… Mwahahahahaha!!!
I’ve fixed that for you…
I’m not sure why this English spelling post has just popped up on the forum again, but here goes:
I always want to spell forest as Fforest.
On a business card, Is the number after the letter “f” the fax number or ffon number?
The village of Hallen in the salt flats near Avonmouth looks strange with its double l.
It all becomes a bit easier in time. Well, for me German spelling is a bit harder then English is and, yes, Welsh is prety easy and logical when you manage to cope with.
Slovene spelling is really easy to spell you spell almost like you hear the words.
I think part of it is what I call “finger macros”, others might say “muscle memory”.
For example, when typing numbers such as “tri ar ddeg”, I keep wanting to write “ar” as “are” because my fingers “autocomplete” the word… I imagine that someone who types a lot of Welsh will get something similar happening in the other direction as well, with Welsh “autocompletions” when the word is supposed to be English!
(Now I wonder whether anybody has ever typed something like “He had to wait there for a longyfarchiadau time” 
)
After thinking about this a bit, I think I have two pronunciations for English (or any other foreign language) words: how they are said and how they are written. The latter meaning that I say a word like it was Finnish one. Makes writing it mostly easy.
My wife has this for loanwords in German - she was strongly affected by dyslexia as a child but found loanwords easier to spell than native German ones because loanwords she had to memorise the spelling of explicitly, e.g. remember that what you pronounce port-mo-nee is spelled *por-te-mon-nai-e" (which she pronounced like that in her head to help her remember the spelling).
(Then along came the spelling reform and proposed re-spelling Portemonnaie as Portmonee to match the pronunciation! Fortunately for my wife, that was in addition to the traditional spelling and not instead of it.)