I agree regarding Welsh, as most Welsh speakers also can speak English. And from the technical standpoint apart from a different field length in files where translated words or sentances may be longer in some instances, there is not the issue of a different character set (save for accented characters) or having right to left rather than left to right.
The issues faced by Monzo will come on the COps side when a language is supported where there is a lower incidence of the speakers being bilingual in English, and on the IT side when a different character set is used such as Russian, Greek and Arabic.
It is far better Monzo look at the issue now than go ahead with a second or third language at this point only to discover later that the app needs a major rewrite to cope with certain languages. Have it able to cope with Arabic and Mandarin etc now and when they go global their won’t be any issues.
They also need to consider their policy on languages. Do they just support the official language of the unitary states they wish to do business in or any official languages of the regions within, e.g. Catalan etc in addition to Castilian, or do they also see what is actually spoken. For example the most common language in Latvia is Latvian, in Estonia is Estonian and Lithuania is Lithuanian, but across the three combines there may be more Russian speakers that there are speakers of one of the national languages. Do Monzo when they provide accounts in those areas provide for 4 languages not three? There are people born in the Baltics who are refused citizenship and a passport until they have a certain level in an exam on the official state language and culture. Provision of the one language but not the other in those countries will clearly show Monzo taking a side in a sensitive political issue and unintentionally supporting the Government and one half of society rather than all the people of the region.
I am sure Monzo don’t want to open a can of worms. They should think about the impact of foreign character sets and languages on their whole operation, IT, COps and marketting. However, from the lack of any contribution from Monzo it looks like they will wait until the issues are staring them in the face rather than preempt and plan in advance what they will do.