visit visa, dutch residency card, sponsorship

[junip]

Hello,

My boyfriend is currently making an application for a short term visitor visa to visit my family with me for christmas.
He is a Belarusian national and has residency in the Netherlands for 8 years, (due for review june 2018.)

1st question:
For the supporting documents, we are wondering what to submit since we are making the application in Czech Republic (because we are currently in Czech it makes sense).
The guide says about additional supporting documents "Confirmation of legal residence, if you are not a national of the country in which you are applying or your right to reside there is not included in your passport"

Therefore is it recommended to give the residency card, or a letter from the city hall, stating his address and right to reside in Netherlands? Also, with the translation, it is not clear to me if the translator should be certified somehow. They say "If you submit a document that is not in English or Welsh, it must be accompanied by a full translation that can be independently verified by the Home Office.
Each translated document must contain:
confirmation from the translator that it is an accurate translation of the original document
the date of the translation
the translator's full name and signature
the translator's contact details

2nd Question:
I am a British Citizen, and my mother will be the 'sponsor'. Does she have to write an invitation letter, or should she fill out this form: https://www.gov.uk/government/publicati ... -form-su07
she will also send bank statements by post so they are original documents.
And should he refer to her simply as "Partners mother". (we are not married or in a civil partnership)

Thank you in advance!