You can never do good for everyone
In the Netherlands (but also in the rest of the world), there are an incredible number of unwritten rules. They are not always told to you, but when you bring it up everyone seems to have an opinion about it. For example, we generally think that we Dutch prefer an informal language over formal. Just take a look at social media and the growing number of websites in your form. But if we receive a letter from, for example, The Tax Authorities, we would rather be addressed in more formal language. Then we suddenly find that distance pleasant. In addition, we also change over time. I used to find it annoying when an intern talked formally to me. Come on, we really don't care that much, did it go through me! Now I laugh about it and I think approvingly 'well it's well behaved'. Okay, so there are no hard and fast rules and you can never do it right for everyone. What do you do if you have doubts?
Wonder what the other person would like
With existing customers, you usually know how they want to be addressed. After all, you know each other. With a prospect, I recommend that you be guided by the conversation. Before you make your proposal, a conversation normally takes place. Physical, by phone or an online. Make enough notes, also regarding the atmosphere of the conversation. Did you quickly switch to tutoying or was it friendly but certainly a bit more formal? Reflecting successful proposals are usually an extension of the conversation. Proposals are also about experience. It would be very strange if you put up a formal proposal in the strict formal form, while your conversational conversation took place in the informal form. The prospect will wonder who he sat down with and there is a good chance that he will begin to doubt.
Is your proposal read by other people? Then discuss it or check your notes. Perhaps you had a tour and the atmosphere was clearly informal. In that case, of course, you can simply choose informally.
Choose what you want to radiate
Sales professionals who work at an organization can usually go back to the corporate identity and branding rules. Corporate identity and branding are reflected in everything, including proposals. If you are an entrepreneur, you decide the style, branding and language use yourself. What suits you, your ideal customer, your services or products? If you have that clear, the choice is made quickly. That's how I made a conscious choice for informal years ago. That's what I like best, and so do my clients. However, if I had a different target group, for example government institutions, I used the formal form just as easily. It's about making you feel good about it. That it's right with what you want to radiate, for whom and for what.
Can't figure it out?
Then just go and practice. Write some paragraphs in both forms. Then read your texts aloud or have someone else read them aloud. You often hear right away when it's not right. Further remember that you are not equal to formal or business and that you and you are something other than popular.
Finally, choose and be consistent
Maybe a no-brainer, but make a choice and be consistent. Personally, I unhook if both formal and informal language are used. It's confusing and distracting. So: choose! Realize that you can never do good for everyone. However, if you make your considerations and choices carefully, apply them consistently and everywhere (including in the terms and conditions of your proposals), you can count the occasional misses on one hand!