Legislation is often complex. It's not unheard of for the legal duty and expected compliance level for public bodies to be different. For TfW the Welsh ministers can direct it to carry out their goals and don't require an instrument of parliament to do so (some exceptions apply). The Welsh Language Measures 2011 clearly sets the intention for Welsh to be treated no less favourably than English. My personal interpretation is while a public body there might not be a legal duty to comply with standards as all the 6 conditions in the duty to comply section are probably not met. They are directed to comply anyway and that ends up as the same result.
The other relevant legislation is the
"Welsh Language and Education (Wales) Act 2025" which requires the Welsh Ministers to target increasing the number of Welsh speakers to 1 million. There is no requirement in either set of legislation not to treat English less favourably. Having everything Welsh first would likely increase the perceived utility of Welsh and increase usage. TfW has a
Welsh language strategy which is Welsh first everywhere. They may not legally be required to have that, but Welsh Ministers could legally require them to if they wished. Relevant quote:
That document also includes reference to developing sector-specific Welsh language standards which implies they don't currently exist. The GBR memorandum of understanding also includes complying and supporting the Welsh language standards. With so few operators it might not need to be in legislation for it to be complied with in practice. There may be areas where legacy tech makes compliance harder and it's a reasonable endeavours situation.
(I don't think there is anything in legislation requiring a private bus or train operator to announce in Welsh first at English stops for cross border services or randomising order everywhere. Traffic signs are different legislation such as Traffic Sign Regulations 2016 with prescribed signs that are Welsh first complete with transition arrangements for older signs. Neither of these are relevant to TfW though)