This subject was comment here https://en.numista.com/forum/topic152881.html#p1201394 but that topic is about other issue.
The guidelines of series are:
- Coins https://en.numista.com/help/series-110.html
- Banknotes https://en.numista.com/help/series-157.html
The rules to apply are the same for coins and banknotes, I copy what refers to coins:
- Circulating non-commemorative coins are usually issued in suites of denominations covering a spectrum of values. For example, from 1 cent up to 2 euros. You may assign the same series name to all the denominations of a particular suite. If possible, name the suite using a distinguishing design or physical feature. Otherwise, name the series after the year when the first coin of the suite was issued. Series should not be named using ordinal numerals. For circulating coins, include the word “series” in the name.
- 16 orbits series || Copper-nickel series || 1991 series
- 3rd series || 3rd issue || 3rd type || Third series
- Use sentence case.
- Copper-nickel series
- Copper-Nickel Series || copper-nickel series
But there are quite a few series that do not follow the guidelines. For example:
- Switzerland
- 1st series ❌ 1907 series
- 2nd Series ❌ 1910 series
- 3rd Series ❌ 1918 series
- 4th series ❌ 1941 series
- 5e série ❌ 1954 series
- 6e série ❌ 1975 series
- 7th series ❌ 1983 series
- 8th series ❌ 1994 series
- 9th series ❌ 2015 series
- United Kingdom
- ND (1946) "1st Series" Issue ❌ ND (1946) series
- ND (1948) "2nd Series" Issue ❌ ND (1948) series
- ND (1956) "3rd Series" Issue ❌ ND (1956) series
- ND (1962) "4th Series" Issue ❌ ND (1962) series
- ND (1972) "6th Series" Issue ❌ ND (1972) series
- United States
- 1814 Issue ❌ 1814 series
- 1823 ❌ 1823 series
- 1826 Issue ❌ 1826 series
- 1850-1859 ❌ 1850 series
- 1862 ❌ 1862 series
- 1862 Issue ❌ Fort Plain Bank 1862 series ?
- 1869 ❌1869 series
- 1969 series ✔️
These series should following the guidelines, or perhaps… should the guidelines be adapted? (for example, use “issue” instead of “series” in these cases).
Also, it is important to fix before translating to other languages.