What Leia said.
cm is ok, since it’s centimetr (sometimes centimedr) in Welsh anyway and it’s also an international (though not, i think S.I.) unit.
Demo is used for demo since there’s no comfortable-sounding abbreviation of arddangos(iad) I would say. It’s also Greek rather than English which matters to some people. 
Thank you @aran, @leiafee and @robbruce. It seams that until now I’m doing just fine what actually makes me happy. I’m using the software anyway so when I finish I’ll have insight into if it’s all OK or something seams odd and needs some corrections. + I have unlimited access to the translating sheet so I can do edits at any time and they’re implemented immediately (when someone starts the software).
Thank you one more time!
Hwyl
Tatjana
Ok…achos and what comes after it…
In school when everything was in black and white it was achos mae hi… (maybe cymraeg byw)…now i’m hearing achos iddi
Help!
SSiW taught me “Dw i ddim isio stopio rŵan, achos mae hi’n ddiddorol”.
Later on phillip
Context? Hearing in SSiW or in normal speech?
Ssiw…can’t remember the phrases…achos iddi beidio … rings a bell
In my memory, we don’t have that in Southern … 
(or I overslept that part ??? )
Actually, it is an almost literal translation of the Welsh arddangos into Latin: de-monstrare - inform/show about
Beth mae’n nhw or Beth yn nhw. I have been getting this one repeatedly wrong on SSIW Southern and wanted to know if beth mae’n nhw was OK, because it has become a bit of a tricky one for me to change.
Depends on context - either can be okay. Don’t worry too much about it, your ear for it will click at some point…
Beth yn nhw’n moyn throws me
That be ‘what do they want’ in lots of contexts…
Ok thanks and the context will cause the switch from yn to mae I suppose or is it always yn?
Yup, this.
Simple examples:
We don’t know what they want - dan ni ddim yn gwybod beth maen nhw isio/moyn.
What do they want? Beth ydyn nhw isio?/Beth yn nhw’n moyn?
diolch - makes complete sense - simple question or not sort of thing.
The use of rhedeg as not just as the physical action of running but also in the context of ‘Dwi’n rhedeg a gym’ as I’ve just heard on Radio Cymru, and multiple other examples. Is the use of rhedeg in terms of running something of a modernish transcribing of the english use in this context or is this the way to run a business has always been described in Welsh?
(And an additional question, is brechdan used that much. what I’m hearing a lot of is ‘sandwich caws’ 'sandwich cyw iar etc)
I’d guess so - there’s a lot of it about.
Yes (or ‘bechdan’ in the north, often enough).
We found it the other way round in Bala - the choice on the chalkboard included ‘brechdan ham’.
Achos mae hi… is technically wrong, and may well have been allowed in Cymraeg Byw, not sure about that. Achos ought to be followed by a ‘that…’ clause, so **achos bod/fod…**etc. Then cases like achos iddi… are the same thing really, with a sense of past time:
Achos bod hi’n cyrraedd yn hwyr
Because she’s arriving late
Achos iddi gyrraedd yn hwyr
Because she arrived late
NOT following it with a ‘that…’ clause is, I think, more common down South, where influence of English is stronger.
Pam Why also strictly speaking requires a ‘that…’ clause:
Pam bod chi fan hyn?
Why are you here?
Preferable to Pam dych chi fan hyn?, which to many native speakers sounds dodgy, and indeed to me as well.